Entomological Alteration of Bloodstain Evidence
Written by M. Anderson Parker, Stacey L. Sneider, Shayne A. Smithey, Mark Benecke, and Jason H. Byrd   

INSECT ARTIFACTS ARE A RESULT of insect activity after the blood from a violent event has been exposed to the environment. It is important to realize necrophagous flies (Diptera) and other arthropods can produce stains and artifacts as a result of feeding on various types of fluids originating from the human body at the crime scene, including blood as well as semen, vaginal fluid, saliva, and decomposition fluids (Benecke & Barksdale 2003; Rivers & Geiman 2017; Rivers & McGregor 2018). These different fluids can yield stains and artifacts varying widely in terms of their shape, color, and size (Fujikawa et al. 2011; Durdle et al. 2013). Any insect or arthropod may leave blood tracks of a diminutive nature simply by walking through wet blood and tracking it onto a nearby surface (Figure 1). In the case of flies, the most common artifact is in the form of regurgitated or defecated blood, and this common pattern is known as fly specks (Figure 2). Fleas (Siphonaptera) can also be responsible for defecating partially digested blood that originated from the victim or others who were present at the scene. Characteristics such as size, shape, and pattern are used by bloodstain analysts to distinguish insect artifacts from legitimate or unaltered blood spatter (Bevel & Gardner 2008).

You’ll find an interview with the editors of Forensic Entomology in this issue of Evidence Technology Magazine.

Figure 1. Mechanical transmission of blood produced by a roach walking through pooled blood and tracking blood onto a clean surface. Arrows indicate tracks made by the rear tarsi of a roach. Areas of larger stain are produced by the abdomen contacting the surface. (Courtesy of Dr. James L. Castner.)

Figure 2. Typical pattern of “fly specs” produced after flies have contacted free blood and fluids of decomposition. These patterns are produced by regurgitate and fecal material produced by flies present at the death scene. (Courtesy of Dr. James L. Castner.)

Although not a true spatter pattern, the fly speck, or fly spot, is often confused with bloodstains created during the commission of the crime. This pattern is created by flies present within the scene feeding on blood, body fluids, and exudates produced during decomposition and tracking (mechanical transmission), regurgitating, or excreting on remote surfaces (Zuha et al. 2008; Durdle et al. 2018; Viero et al. 2018). In the instance of the tracking pattern, the marks are extremely small, typically 0.5–1.0 mm, and a pattern may be evidence on close examination (Figure 3).

Figure 3. On close examination, the regurgitate pattern is roughly symmetrical, and the fecal material is an elongated “comma” shape. The randomness of the pattern and contrasting directionality of the stain indicates that this pattern is produced by insect activity and is not a resulting bloodstain from a violent act committed during the crime. (Courtesy of Dr. Mark Benecke.)

In the case of fly regurgitation, the specks are remarkably symmetrical and are sometimes lighter in color than surrounding blood drops. In the past, it was suggested that these patterns were usually found in brightly lit and warm areas where the flies rest, such as high in window corners, along walls where the sun strikes, the back side of blinds and curtains closest to the window, and on objects near windows. (Figure 4a,b). However, in a study conducted by Durdle et al. (2018), whether Calliphora erythrocephala (Diptera: Calliphoridae) moved toward or away from the light is dependent on the age of the flies. Younger flies tended to move toward the light due to increased flight activity, whereas older flies tended to move away from the light due to more time on the ground and near the food source. This means that crime scene examiners need to be aware of the presence of fly artifacts in all areas of the crime scene, not just warm or well-lit areas. They also determined a strong correlation between the number of artifacts per fly found at the completion of the experiment and the maximum temperatures at the scene.


Figure 4. Differences between bloodstains caused by violence in contrast to artifacts caused by flies. (a) Stains produced by flies on a vertical surface (sheet of kitchen paper). Note random orientation of tails, difference in width of body and tail indistinct, tails do not end in satellite spot. (b) Complex bloodstain pattern caused by blood that exited a punctured blood vessel of a man who was standing next to a vertical plane (wall). Larger stains show orientation, body and tail of droplets easy to distinguish, tail ends in satellite dot. (Courtesy of Dr. Mark Benecke.)

Flies tend to rest or walk at low temperatures rather than fly. The overall results of the experiment highlight the fact that the location of the fly artifacts found at the crime scene will neither be limited to the ceilings or walls, nor will they only be concentrated around food or light sources. Flies may move human DNA from one location to another, particularly in climatic conditions during which they are most active. Adult flies can move great distances after feeding and before defecating and regurgitating the fluids they have ingested (Durdle et al. 2013). This knowledge is particularly important considering that 48–80 hours can be required for a meal to pass through the fly digestive system. While the time it takes for a fly to digest a meal depends on a number of factors such as ambient temperature (Muntzer et al. 2015), the concentration and type of food, and the level of hunger, this shows that there may be ample time for a fly to travel to a location distant from the food source or even crime scene before depositing an artifact that will most likely contain DNA. This knowledge of the fly’s behavior will ensure crime scene examiners are aware of the variable distribution of artifacts so that an ambiguous spot of blood is not dismissed as unlikely to be a fly artifact due to its location in an “unexpected area.” It is also important for investigators to recognize that fly contaminants are not restricted to the primary crime scene or room. The fact flies display this positive phototaxis (depending on age) and thus will be attracted to any accessible windows and lights throughout and near the main scene should be kept in mind when searching for artifacts or when these artifacts are observed in other areas falling under these “guidelines.”

Both regurgitation and defecation stains will usually test positive for blood with a presumptive test. It is critically important for the investigator to understand that the artifacts caused by the flies include the victim’s blood. Therefore, presumptive tests like Hemastix (2190)/Heglostix [Bayer 028165A; hemoglobin catalyzes oxidation of 3,3,5,5-tetramethylbenzidine (color reagent) by diisopropylbenzole dihydroperoxide from green to blue], Sangur (Merck), or Luminol will not differentiate between the two types of stains. Additionally, DNA typing will not differentiate between the two types of stains. In some instances, full DNA profiles have been obtained two years after deposition. In fact, when adult flies regurgitate and defecate some of the ingested food onto surfaces at or near the original crime scene, the food can be an intermixing of not only blood, but other human body fluids (Benecke & Barksdale 2003; Fujikawa et al. 2009). Regardless of the precise composition, stains resulting from regurgitated body fluids, including blood, are expected to be chemically similar, yet still distinct from the original food source or sources (Durdle et al. 2015; Rivers et al. 2018). Since these common laboratory tools are useless, this leaves recognition of stain patterns and other physical information as the relevant criteria. Any method of visual and/or contextual analysis of fly artifacts relies on the experience and opinion of the analyst rather than on standardized and reproducible methodology. But contextual analysis does not overcome limitations of visual analysis in some cases, especially when the insect stains lack tails (Figure 4a,b), are intermixed with human body fluid stains, and/or the artifacts are of similar size to the body-fluid stains. Obviously, care should be exercised in evaluating any abnormal patterns that meet these criteria.

The inability to consistently and reliably distinguish insect artifacts from human bloodstains and body fluids represents a serious deficiency when addressing entomological contaminants at crime scenes. Several attempts have been made to find a method to differentiate fly artifacts and bloodstain patterns at the scene with modest success. However, none are satisfactory based on limitations that include lack of repeatability and reliability, no single pattern can be attributed to all fly species, and no consistent visual distinction of fly regurgitate and fecal spots from other forms of human body fluids that may also be present at crime scenes. Additionally, all forms of visual analysis for insect alteration of bloodstains are presumptive and not confirmatory. One example is Fujikawa et al. (2009) who determined an alternate light source (ALS) may assist in identifying insect artifacts and bloodstains by examining each under a wavelength of 465 nm with orange filter goggles. The result was that only some defecation statins fluoresced and no regurgitation stains fluoresced. The inconsistency of the fluorescence is not reliable enough to be used to distinguish fly artifacts from bloodstains in all cases, although it may be a means to assist in some circumstances. Rivers et al. (2018) demonstrated regurgitate stains deposited by Protophormia terraenovae Robineau-Desvoidy (Diptera: Calliphoridae) possessed at least three digestive enzymes that were also found in the crop of the adult fly, independent of the food source. The study determined the polyclonal antisera Anti-md3 serum reacted to both regurgitate and defecatory stains, but not to any type of mammalian blood tested alone (Rivers et al. 2018). The serum demonstrated a high degree of specificity for fluids and stains containing cathepsin D-like proteinase, a digestive enzyme found in the foregut of the flies used in the experiment. The antisera reacted with both regurgitate and defecatory stains, but they were not able to distinguish between the two. This is considered a minor weakness because the primary need is to differentiate fly artifacts from human bloodstains in a reliable, quantifiable manner. The antisera did not bind to stains produced by translocation or tarsal tracks. This represents a significant step toward the development of a confirmatory test that allows distinction of fly artifacts from human bloodstains. Unfortunately, utilizing the anti-serum in confirmatory testing is also dependent on the recognition of artifacts produced by several species of forensically important flies. At this time, the enzymatic profile of other species of adult necrophagous flies is not known; however, additional work is underway on this topic. There are also additional field studies being planned to address the topic of other body fluids found at the scene, as well as utilizing these methods under field conditions as opposed to the current lab conditions only.

Another consideration is that since fly artifacts can yield the DNA of the source material, this presents an opportunity to use fly artifacts as a source of DNA from the scene. If the scene has been cleaned, or if original stains are no longer available, fly artifacts found in remote areas of the scene can yield valuable genetic evidence. This evidence can be useful to the investigator in assisting in placing a victim or suspect in a location where the original scene may have been compromised. The investigator must also keep in mind that in these discussions of fly artifacts caused by regurgitate and defecation, the assumption is that the adult flies depositing the artifacts have consumed only the blood and tissue of the victim, or possibly suspect, found at the crime scene. As stated earlier, regurgitated and defecated artifacts are a result of insect activity after blood or body fluids have been exposed to the environment, and this can be any blood or body fluids they have access to prior to landing on a surface inside, or in close proximity to, the crime scene. It is not uncommon for flies to be introduced to the corpse inside a crime scene during the initial call taken by the first responders or during the actual crime scene investigation. These flies can gain access through doors opened by the first responders, windows opened to air out the scene for odors or extreme temperatures, or by the comings and goings of detectives and crime scene investigators through the doors of the interior scene. In such scenarios, deposition of regurgitate and/or defecation that is chemically distinct from the fluids or tissues found at the crime scene could be introduced through these new access points. The investigator must keep in mind that flies introduced to the crime scene by these means could deposit fly artifacts containing DNA from an individual who is in no way associated with the crime scene they are examining.

The investigator must have a firm understanding of the important information to be gained in the analysis of bloodstain evidence. The information obtained by bloodstain pattern analysis can be used for the reconstruction of the incident and for the evaluation of other important issues related to the death, such as the positions and movements of both the victim and perpetrator both during and after the bloodletting event, as well as the assessment of the statements made by witnesses. This information can include what did (or did not) take place and answer the question of who may have been involved in these actions. It is an unfortunate truth that in most crime scene investigations, it is highly unlikely that a bloodstain pattern expert will actually enter the scene. Therefore, most analysis is done by the expert while reviewing the documentation of another investigator’s work. Within our experience, the proper documentation of bloodstains at the scene usually does not take place. Too often, lack of proper documentation due to too few photographs or the photographer taking photographs too far away from the patterns makes it so that the relationship of one stain to another is anything but evident in the crime scene photographs. This does not need to be the case. Proper documentation is simple and when combined with the roadmapping method, it eliminates much of the viewer’s confusion and also makes for outstanding and useful case documentation. The roadmapping method was developed by Toby Wolson, a criminalist with the Miami-Dade Police Department in Miami, Florida. It allows overall patterns as well as individual stains to be documented using a series of scales that takes the viewer from an overall view of the pattern in the scene to each individual stain that the analyst feels is important within the pattern while keeping the relationship evidence between each using a series of letter and number labels. These methods will be discussed later in the chapter.

In our experience, the following suggestions and techniques are offered for use in differentiating between fly artifacts and human bloodstain patterns (Figure 4a,b):

1. Document any fly activity at a scene with photographs and written notations. Flies will be at the scene if access to the scene is available. They will stay at the scene as long as a food source is available or as long as they are trapped. Bloodstains can be altered after formation, especially in the liquid phase. The flies will continue to create additional, spatter-like patterns as long as they have access to liquid blood at the scene due to mechanical disruption, regurgitation, and defecation. Therefore, check for and document any dead flies that may be present. If evidence of flies is present at the scene, assume that fly artifacts will be at the scene. Follow standard protocols for insect collection as outlined in Chapter 3.

2. Document the areas in which stains are found both photographically and with written notations. Fly activity will often be found to concentrate near light sources, on light colored walls, and near windows and mirrors. It is also common for them to be present in rooms located well away from the body. Make a photographic comparison between stains away from the body and stains near the body. The presence of blood spots far from the sources of blood suggests that the stain is a fly artifact versus a bloodstain because the flies are able to deposit blood mechanically as well as leave fly artifacts in rooms where blood is not present. The stain and location in context with the crime should assist with determining whether it is from the bloodletting event or from insect activity.

3. It is of critical importance for the investigator to compare stains at the scene with known fly artifact patterns. In many instances, the patterns produced by insects are remarkably consistent.

4. Identify suspected human bloodstain patterns that are of the spot or teardrop pattern that offer a potential use for reconstruction and eliminate the following indicators of entomological origin:

a. Stains that have a tail/body (Ltl/Lb) ratio greater than 1

b. Stains with a distinct head and tail shape (tadpole-like in silhouette)

c. Stains with a tadpole-like shape that do not end in a small dot

d. Any small stains (less than 4 mm) without a distinguishable tail and body, with a clear, white, or yellow central area

e. Any stains with a wavy and irregular linear structure

f. Any stains that do not have principal directionality consistent with other stains, which suggest a common point of convergence, or a common point of origin. Within a large grouping, bloodstain fly artifacts will point in all directions. However, cast-off blood from human activity will produce stains that within a group have a common general convergence point.

Probably the most important observation for an investigator to make is to note the absence of known human bloodstain pattern characteristics. For instance, the absence of misting around a concentrated mass would suggest that stains might not be from impact or cast-off. Within a group, human castoff patterns often leave secondary wave cast-off patterns and runoff patterns. As with any investigation involving bloodstain evidence, it must be kept in mind that one or two stains do not make a case. Stains that could be fly artifacts should be eliminated and an evaluation made based upon stains that can be in terms of origin and relevance to the reconstruction. As with general crime scene investigation techniques, always use a high-resolution camera and a macro lens capable of 1:1 reproduction. Always take duplicate photographs with and without a photographic scale shown with the evidence.

Other ongoing documentation will be general scene photography, videography (when applicable), and sketches. Sketches are useful for visual interpretation of blood-evidence notes without the distractions of non-evidentiary objects that may be present in the photographs. Proper lighting and light angles are important in scene photography, as the photographs may be used for further analysis and scene reconstruction. If a department does not have a bloodstain pattern analyst or training in analyzing bloodstain patterns, outside assistance will be needed. The analyst assisting the department will most likely not be able to respond to the scene and will have to rely on the documentation of the scene and of the stains to conduct their analysis. It is also important that the bloodstain pattern analyst is able to present their opinions and reconstruction to the courts in a way that is clear and understandable. The analysis process requires detailed photographs, but sometimes the person tasked with taking the photographs at the scene may not understand the patterns or what is important to document. If, however, the photographer is able to follow a fairly simple set of instructions, they should be able to document the scene in a manner that will assist in successful analysis in the future.

Bloodstain pattern documentation follows the same rules as general scene photography in that the photographer will focus on overall photographs first, then midrange and establishing photographs, and finally, close-up photographs. This method works with all types of stains. By documenting each overall pattern and then moving to individual stain details within the pattern, the viewer of the photographs is never lost as to what they are looking at. The first step is to document the entire scene as found with 360° photographs of each of the rooms, including the ceiling and floor. There should be overlap between each of these photographs. The photographer then needs to identify the patterns on each of the surfaces in the room and place a vertical and horizontal scale around the pattern. Each of the patterns is then labeled with a letter or number (A, B, C, etc. or 1, 2, 3, etc.). Stains of interest within each pattern are marked with a labeled scale with the pattern designation and then a stain designation (A1, A2, A3, etc.). The photographer takes overall photographs with the labels and scales placed into the scene to show the orientation of the patterns to others as well as to orient each individual pattern in the scene. The next step is to take a midrange photograph of each of the individual patterns, aligning the framing scales along the edge of the photograph and filling the frame with the patterns and keeping the lens of the camera parallel to the surface being photographed. The purpose of the midranges is to show the overall entire pattern, even if some of the finer detail within the pattern is not obvious in this shot. The midrange photograph is taken from whatever distance is necessary to show the entire pattern in the frame. The final step is to take close-up photographs of each stain of interest within the pattern, along with its labeled scale. It is recommended that a macro lens is used to fill the frame with the stain and capture a clear, undistorted image of the individual stain. Remember to keep the camera lens parallel to the surface on which the stain in located. This will allow another analyst to measure the stain using the photograph if an Area of Origin determination is necessary. Another suggestion is to add a plumb-bob line to the scale in case the analyst has access to software that can assist with calculating areas of origin. The labeled scale must appear next to the stain in order for the stain to retain an identity. A photograph with no landmark is of little value and makes it hard for the viewer to determine what stain they are looking at or even what the proper orientation of the stain is if there is no label. These steps are important to follow because without evidence establishing photographs, patterns and stains lose context, particularly when viewed by others later in the investigation. These photographs are a way for the investigator to present the scene to someone who was not there and present the scene the way the photographer saw it.

Roadmapping is the most effective method of completing photographic documentation of bloodstains. One thing to keep in mind, however, is although it is very effective, it is also intrusive. To properly map the scene, the photographer must insert many “road signs” and scales for reference. Because of this, the investigator should wait until the completion of the scene processing to complete the roadmapping. The process is very similar to the basic photographing of patterns that was just described above. Each major stain group is identified with a label, either a letter or a number. The individual stains or smaller patterns of interest within the major stain are given an additional identifier for example, A1 and A2 in stain A. Labeled scales are placed by each stain of interest with large lettering on each scale that allows them to be read from several feet away. Large reference scales are placed adjacent to the pattern on the surface involved, framing the major stain group. A minimum of two reference scales are required, one along the vertical axis and one along the horizontal axis. If needed, the whole pattern can be surrounded by the reference scales. The photographer then photographs the whole surface, including the reference scales. The stain labels should be readable in these shots. The photographer then moves in and photographs the individual patterns. Next, the photographer moves closer and photographs the individual stains within the patterns. In any given photograph in this method, the labeled scale should make it evident where on the wall or surface the analyst is looking and at what pattern. These labels are the “signs” on the roadmap; they keep the viewer from being lost. Roadmapping is an effective way to take the viewer from an overall view of the scene, to the individual pattern, and then to each individual stain. This method works not only at the scene, but can also be effective when documenting individual pieces of evidence.

The first step in the actual analysis is to determine, if possible, the bloodstain patterns from any created artifacts. In order to accomplish this, it is necessary to be familiar with the terminology used in this discipline. At the 2009 International Association for Bloodstain Pattern Analysts (IABPA) meeting in Portland, Oregon, the IABPA membership voted to adopt the Scientific Working Group on Bloodstain Pattern Analysis (SWGSTAIN) terminology as its recommended terminology. Some bloodstain pattern analysts have created their own classification methods as well. In Bloodstain Pattern Analysis: With an Introduction to Crime Scene Reconstruction, Tom Bevel and Ross Gardner use a taxonomic classification system where the main categories are The Spatter Family, The Non-Spatter Family, and Complex Patterns. In Principles of Bloodstain Pattern Analysis: Theory and Practice, Stuart James, Paul Kish, and Paulette Sutton have used the categories Passive, Spatter, and Altered. In Bloodstain Pattern Evidence: Objective Approaches and Case Applications, Anita Wonder provides a flow diagram to assist in classifying bloodstain patterns where the main categories are Spatter Groups, Spatters Not a Criteria, and Composites. Some of the most important and basic terminology used to define bloodstain patterns are included in this chapter. The following are some of the most common pattern definitions, while more complete lists may be found on the SWGSTAIN website.

Altered Stain: A bloodstain with characteristics that indicate a physical change has occurred

Backspatter Pattern: A bloodstain pattern resulting from blood drops that traveled in the opposite direction of the external force applied; associated with an entrance wound created by a projectile

Bloodstain: A deposit of blood on a surface

Bloodstain Pattern: A grouping or distribution of bloodstains that indicates through regular or repetitive form, order, or arrangement the manner in which the pattern was deposited

Cast-Off Pattern: A bloodstain pattern resulting from blood drops released from an object due to its motion

Drip Pattern: A bloodstain pattern resulting from a liquid that dripped into another liquid, at least one of which was blood

Expiration Pattern: A bloodstain pattern resulting from blood forced by airflow out of the nose, mouth, or a wound

Flow Pattern: A bloodstain pattern resulting from the movement of a volume of blood on a surface due to gravity or movement of the target

Impact Pattern: A bloodstain pattern resulting from an object striking liquid blood

Insect Stain: A bloodstain resulting from insect activity

Projected Pattern: A bloodstain pattern resulting from the ejection of a volume of blood under pressure

Swipe Pattern: A bloodstain pattern resulting from the transfer of blood from a blood-bearing surface onto another surface, with characteristics that indicate relative motion between the two surfaces

Transfer Stain: A bloodstain resulting from contact between a blood-bearing surface and another surface

Void: An absence of blood in an otherwise continuous bloodstain or bloodstain pattern

Wipe Pattern: An altered bloodstain pattern resulting from an object moving through a pre-existing wet bloodstain.

It is important to note that by definition, only one type of insect artifact is officially recognized in this terminology list, insect stains. But this definition leaves open the possibility of producing those insect stains by two methods: insect modification of an existing stain as well as the creation of new stains from insect activity.

As stated previously, a single drop of blood does not identify the pattern. The entire area of bloodstains must be evaluated. However, a single drop must not be overlooked, as it could be an important piece of bloodstain evidence. The overall pattern may indicate what happened where, how, and when, but a single drop may provide the who part of the who, what, when, where, and how equation.

It is not always possible to accurately identify a pattern, due to conditions and circumstances too numerous to list here. An analyst examines all aspects of the blood evidence available in order to make a pattern determination. The results of these examinations should be included in the scene notes and reports, even if a pattern cannot be identified.

Since each pattern at a scene reveals certain unique information, a brief discussion of each of the previously listed patterns is warranted. The cast-off pattern when made by the instrument of impact attack will tell the minimum number of blows struck. The first blow will draw blood that adheres to the instrument, and the subsequent blows will create the pattern. The blood drops in this pattern will generally be linear in appearance (Figure 5). A drip pattern may indicate the location of a blood source that did not move for a period of time (although the timeframe is difficult or impossible to determine), and a pool of blood with satellite blood drops characterizes this pattern (Figures 6 and 7).

Figure 5. Typical “cast-off” pattern produced by a human hand covered with blood. (Courtesy of Dr. Mark Benecke.)

Figure 6. Typical drip pattern created when blood from a stationary source drips into previously pooled blood producing satellite spatter indicated by arrows). (Courtesy of Tallahassee Police Department.)

Figure 7. Pattern produced when stationary pooled blood is impacted by an external force. In this image, a hammer has impacted a pool of blood. (Courtesy of Dr. Mark Benecke.)

There are three types of impact patterns: high-, medium-, and low-velocity impact. Each is typically defined by the diameter size of the majority of stain drops, although stains of all sizes may be present. There are certain, but not always, inclusive actions that may be associated with each type. High-impact stains have the smallest diameter (0.1 mm or less) and are mist-like in appearance. Such patterns are produced when blood is exposed to a force of 100 feet per second or greater. However, bloodstains from high-velocity impacts are typically associated with gunshot wounds and sneezing and coughing of fluid that contains blood (Figures 8 and 9). However, such patterns may also be produced when trauma is induced by either heavy, blunt objects or rapidly moving objects. This type of trauma and the resulting blood patterns are often produced in industrial accidents. Insects present at the crime scene can also create droplet-sized stains such as these (Striman et al. 2011). Often the mechanical transmission of blood from a pooled source to a blood-free surface is accomplished by the tarsi (feet) of flies or the fecal material of fleas (Figure 10a,b). Roaches can also produce apparent blood droplets by their blood-contaminated tarsi, but the droplet size is larger than that produced by fly tarsi.

Figure 8. Impact stains and other patterns produced by coughing blood. (Courtesy of Tallahassee Police Department.)

Figure 9. High-velocity impact pattern produced by a gunshot wound. (Courtesy of Tallahassee Police Department.)

Figure 10. (a) “Flea specks” are a common pattern produced by insect activity that is often confused with a pattern that may be created from a gunshot. Fleas deposit small droplets (less than 1 mm) of blood when they feed. This image shows the deposition of blood droplets as fleas feed on a human host. (Courtesy of Dr. Nancy Hinkle UGA.) (b) “Flea specks” found on an apartment wall at the scene of a bludgeoning. The large stain in the center was produced from the attack. The smaller droplets, initially thought to be produced from a possible gunshot wound, are flea specks. (Courtesy of Tallahassee Police Department.)

In the case of roaches, such artifacts are usually easily distinguished, as the overall droplet pattern reveals a series of “tracks” that often have a center drag mark or smear caused by the roach dragging its undersurface or the abdomen tip as it walks (Figures 11 and 12). It is this intermittent smear that first draws attention, and the imprints made by the roach’s tarsi are usually apparent upon closer inspection (Figures 13 and 14). In many cases, such details are best viewed with a hand lens. With flies, such an event is not as easily distinguished since the pattern produced is more isolated, smaller, and not as uniform. Flies with bloodsoaked tarsi may alight on any surface, vertical, horizontal, or inverted. Once on the surface, the fly may not walk, or not all of the legs may be contaminated with blood, thus producing no discernible tracks or repeatable patterns. Additionally, the fly may not alight with all six tarsi in contact with the surface. Flies often rest on only the hind two pairs of legs, using the first pair to clean the eyes and head (Figure 15), or they may rest on the first two pairs, using the hind pair to clean the abdomen. Thus, the typical six-point track of an insect is often not discernible. When a fly with blood-contaminated tarsi walks on a surface, a repeatable pattern can be produced, but no drag mark or smear occurs since the adult fly does not drag its abdomen as it walks. Therefore, such insect-produced blood droplets may be difficult to distinguish from high-velocity droplets produced during the commission of the crime.

Figure 11. Transfer pattern produced by adult roach, Periplaneta americana, running through pooled blood. Note the elongated trail produced by the roach dragging its abdomen. (Courtesy of Dr. James L. Castner.)

Figure 12. Transfer pattern produced by adult roach, Periplaneta americana, slowly walking through pooled blood (Courtesy of Dr. James L. Castner.)

Figure 13. Enlargement of Figure 11, showing the drag pattern produced by the roach’s abdomen and tarsal imprints. (Courtesy of Dr. James L. Castner.)

Figure 14. Bloodstain artifacts produced by a roach moving through pooled blood. Note imprints of abdomen and distinct tarsal tracks (indicated by black arrows).

Figure 15. All legs of an insect are not always in contact with the surface. Insects groom frequently and do so by rubbing the body with their legs. This behavior often produces irregular artifacts in bloodstain evidence. Here, Sarcophaga bullata (Parker) cleans its compound eyes, leaving only four legs in contact with the surface.

Many of these droplets created by insects will be found in patterns of three pairs or readily distinguishable pairs of two. Often these tracks will be visible as a linear pattern of three pairs of droplets with each row separated by 0.5–1.75 cm with flies and 2.0–3.0 cm with roaches. Such tracks will only span a short distance due to the small amounts of blood that typically cling to the small tarsi of flies and due to the fact that flies do not usually walk far before once again taking flight. These patterns most commonly occur on vertical surfaces (such as light colored walls) and on ceilings because of the behavioral propensity of flies for landing on these surfaces.

Medium-velocity impact stains range in size from 1 to 4 mm diameter (Figure 16) and are produced by a force other than gravity of 5–100 feet per second (1.5–30 mps). They are most commonly associated with blunt-trauma injuries. Many artifacts produced by the presence of insects overlap with this droplet size class as well as the low-velocity impact size class. Regurgitated and defecated fly artifacts are generally 1–2 mm in diameter. Low-velocity impact stains have the largest diameter, 4 mm or larger, and are formed when blood is exposed to a force of approximately 5 feet per second or less. These are usually associated with gravitation force and may form part of another pattern, as illustrated in Figure 17. Within the medium-velocity size class exists certain patterns that can be immediately recognizable as produced by (or associated with the presence) of flies. Fly specks are the term entomologists give to the deposited liquid fecal material of flies. In a study done by Durdle et al. (2013), fecal artifacts were found to outnumber regurgitated artifacts by a ratio of 30:1 to 95:1. This material contains large volumes of partially digested and undigested blood if the adult flies have recently fed on free blood or a body at a crime scene. Blood ingested by flies is not completely digested before defecation and may even pass through the system without any degradation. Thus, a fly feeding on human blood will pass fecal matter with a large amount of partially digested blood that resembles blood from both the biomolecular and chemical points of view. This fecal matter will test positive as human if a presumptive test is conducted at the scene. Blood contained within these droplets can be collected and used in all standard laboratory molecular and blood serology tests. Drops such as these may be distinguished from those created during the commission of a crime by their overall shape. These stains are varied in terms of shape and color based on the species of flies depositing them. Eliminated feces on smooth surfaces commonly appear round or asymmetrically round, sometimes with tails and, on other occasions, they may appear linear or sausage shaped. Defecatory stains can also be found in shapes that are described as sperm- or tadpole-shaped, teardrop shaped, or comma shaped. The tails on these stains can originate from the fly beginning to walk prior to the completion of defecating, which produces a tail tapered in the direction of the fly movement, or as a result of forcible expulsion of the liquid feces from the fly’s protruded anus. Rivers et al. (2018) have found in their study that with some species, Lucilia sericata (Meigen) or Calliphora vicina (Robineau-Desvoidy) (Diptera: Calliphoridae), these tails are reported to be absent from defecatory spots, making these flies’ fecal and regurgitate stains indistinguishable from each other. This finding underscores attempts to identify defecatory stains based on the presence of tails since some species may never or rarely deposit fecal artifacts with tails. Fly specks are a swipe pattern that typically exhibits a comma shape, with the tail of the drop trailing to the left, right, or straight of center, depending on the movement of the fly abdomen (Figure 18a–c). The fly touching the tip of the abdomen to the surface as it defecates and walks about produces this type of bloodstain artifact. It has the same basic characteristics as many other swipe patterns and can be easily recognized by the trained analyst.

Figure 16. A typical medium-velocity impact pattern. (Courtesy of Melvin Bishop.)

Figure 17. A low-velocity impact pattern (with other patterns) on a wall and baseboard. (Courtesy of Tallahassee Police Department.)

Figure 18. Characteristic blood imprints of fly fecal spots, or “flyspecks,” with tail curved to the left (a), right (b), or center (c). Swipe patterns such as these can be easily recognizable to the trained analyst. (Courtesy of Dr. James L. Castner.)

Flies also will produce medium to large droplets due to their natural feeding behavior and digestive habits. As an adult fly digests its liquified meal, it frequently regurgitates its gut contents. This regurgitation is considered to be the expulsion of food from any location within the foregut out the oral opening. The regurgitation accumulates as a medium to large droplet at the tip of its sponging mouthparts (Figure 19). Often this drop either falls to the surface or is accidentally touched to the surface. Upon contact with a surface, the drop usually adheres or is absorbed and is only partially reconsumed by the fly. The drop is usually symmetrical and has smooth, circular borders (Figure 20). The droplet itself can be highly variable in color and may reflect the food consumed by the fly. The fate of the regurgitated droplet appears to be dependent on the meal consumed by the fly, the fly species, and whether the adult is disturbed during the deposition of the droplet, which can result in a teardrop shaped stain. If the regurgitate is touched to a non-absorptive vertical surface, it may also produce a tail descending straight down the midline and lower edge of the droplet, but this is not always the case due to the viscous nature of the regurgitate.

Figure 19. A female Cochliomyia macellaria blowfly with a drop of regurgitate on the sponging mouthpart. This is a common feeding behavior found in all species of Calliphoridae, Muscidae, and Sarcophagidae. (Courtesy of Steve Grasser www.bugwood.org.)

Figure 20. Typical symmetrical bloodstain produced by the regurgitation of blood by an adult fly. This regurgitate spot was deposited on a vertical plaster surface by Phormia regina (Meigen). (Courtesy of Dr. James L. Castner.)

When an impact pattern is created, the blood drops generally form a radiating pattern, with the highest concentration in the center, but seldom in a complete circle. From this pattern, the analyst may calculate the impact angle, which is defined as the internal angle at which blood impacts a surface, and use the results to determine the point of origin of the blood drops. The mathematical formula utilized to calculate the impact angle is relatively simple. First, select representative and well-formed stains within the pattern. Of these stains, each one will be calculated individually. Measure the width (W) of the stain and divide by the length (L) of the stain. Using any scientific calculator, simply compute the arc sine of the resultant value (angle of impact = arc sine W/L). The result will be the impact angle, and these measurements can then be utilized to calculate the point of origin. The results are used to form a three-dimensional view that provides a range of height and distance of the blood source from the impact surface. Some of the steps used to determine this are depicted in Figures 21 and 22.

Figure 21. Blood drops being charted to determine the point of convergence. This step is an integral component of determining the point of origin. (Courtesy of Tallahassee Police Department.)

Figure 22. The three-dimensional view revealing the height and distance ranges of the blood source from the impact surface. This is the final step in determining the point of origin to reveal the height and distance ranges. (Courtesy of Tallahassee Police Department.)

The passive pattern may be used to show movement. A trail of passive drops will indicate the blood source’s direction of travel, or the flow may show movement of a body after death. The most common instance occurs when the subject bleeds from the nose or mouth, as seen in Figure 23. The blood will flow from the orifice to the lowest point, independent of the body position.

Figure 23. Passive flow from the nose, ear, and head wound in a shooting suicide case. Note the void in the blood pool created by the victim’s face and chest. (Courtesy of Tallahassee Police Department.)

Projected blood patterns are commonly associated with arterial spurting or gushing. Such patterns may also be created by the vomiting of blood and are not usually duplicated through insect activity. Movement of a person or object through pooled blood may cause the blood to project, which is characterized by a large volume of blood with spines. On a vertical surface, it appears as a large upside-down drop with a drip. This phenomenon is illustrated in Figure 24, from a shooting case and a dismemberment/arson case, respectively. Larger insects (such as roaches) walking through areas of pooled blood will alter the projected pattern, and typically produce pronounced tracks. Fly larvae, also known as maggots, in the families Calliphoridae, Sarcophagidae, and Muscidae are the primary consumers of animal organic matter. After completion of their feeding phase, the majority disperse to find an adequate place for pupariation and then pupation. A small amount of the maggots will stay on the corpse and in the clothing. These wandering larvae are called “post-feeding” because they stop feeding and then empty their digestive system of any food remains. These post-feeding larvae can travel up to 10 meters away from the body to surrounding sheltered areas. If the larvae are migrating from a surface that is soaked or covered in liquid blood or putrefactive liquids, they could produce a series of trails, linear wipe patterns, as a result of their crawling motion. Therefore, it is important to note what types of insects were observed at the crime scene and to collect the insects properly. Chapter 3 addresses the proper insect collection protocol at a crime scene.

Figure 24. Victim was shot through the nose and projected blood on the wall and door when he exhaled. (Courtesy of Tallahassee Police Department.)


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Durdle, A., R. Mitchell, and R. Oorschot. 2013. The human DNA content in artifacts deposited by the blowfly Lucilia cuprina fed human blood, semen, and saliva. Forensic Science International. 233: 212–219.

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Rivers, D., and A. McGregor. 2018. Morphological features of regurgitate and defecatory stains deposited by five species of necrophagous flies are influenced by adult diets and body size. Journal of Forensic Sciences. 63: 154–161.

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This article appeared in the Fall 2019 issue of Evidence Technology Magazine.
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