Use of Dermestid Beetles for Skeleton Preparation
 
By
Stephen H. Hinshaw
Coordinator of Museum Collections
Mammal Division
University of Michigan Museum of Zoology

The use of Dermestid beetles has a relatively long history in American museums. One of the first references to the use of Dermestid beetles in the preparation of skeletons for scientific study was by Ward Russell in the Journal of Mammalogy, Vol. 28, pp.284-87, 1947. Much of the information included here can be found in Russell’s paper, however, there are significant differences in a few areas.
 
There are many other ways of cleaning skeletons, such as terrestrial isopods, marine isopods, soil maceration, water maceration, enzyme detergent soup, manure pile maceration and others. These all have their strengths and weaknesses and any of them may be the best method at a particular time in a particular place for certain specimens. Still, dermestids remain the most common method employed at most museums.
 
If you look carefully at the skeletons in the collections at the University of Michigan Museum of Zoology there is a noticeable shift in the patina on skeletons prepared after approximately 1937 when the museum began using dermestid beetles for skeleton preparation. The macerated skeletons are considered by some curators to be more thoroughly cleaned than bugged skeletons, but dermestids are much faster and specimens require much less preparator time.

 

STARTING A DERMESTID COLONY

In most areas dermestid beetles are common and easily found by checking a few carcasses of roadkill or other carrion. A single gravid female can start a colony, but obviously the more that can be found to start with the faster the population will grow and the faster they will clean skeletons. We use one of our local species, Dermestes maculatus,because they are common, easy to find, and most importantly, have biological attributes that make them ideal for year-round captivity eating flesh and connective tissue.
 
Be sure that you are bringing only dermestids into your colony. Check them over with a magnifying glass or dissecting microscope for the presence of mites, Lardoglyphus zacheri,under the elytra or adhering to the side of the thorax, the spiracles, or the head. If you find mites, either look for mite-free beetles elsewhere, or if desperate, you can purportedly pick off by hand all of the mites on a few individuals to start a colony.

 

DERMESTID BIOLOGY

There are many carrion beetles in most parts of the world that are part of the successional carrion fauna that feeds on carcasses. However, there are some important characteristics that make the genus Dermesteswell suited for use in museum skeleton preparation. In most North American museums, a local species of Dermestesis favored for skeleton preparation. Dermestes maculatusfor example, works well in museums because they cannot fly at temperatures below approximately 80ºF, but reproduce and thrive at room temperature. Since they can’t fly at room temperature it is much easier to work with the colonies, putting in carcasses, taking out skeletons, moistening their wet towlettes (You think I’m kidding, but I’m not. More on that later.), and other dermestarium chores. At the same time, they are extremely active and live well and prosper in the alien world of the museum colony. The following chart shows a brief sketch of the Dermestes life history.

Stage of Development Time Interval Feature
Egg 3-4 Days Creamy white, 2mm long
Larva 44 Days Averages 7 molts
Pupa 7-8 Days Last larval skin serves as protection
Adult (preoviposition) 10 Days Male beetle smaller than female
Adult (oviposition) 2 Months at least Weak flier, fast runner
Adult - total life span 4-5 Months  

The most important aspect of their biology relative to our use is their size. They are ideal in that they are large enough to eat flesh quickly, especially in a large museum colony, and yet small enough in the early larval stages to clean the smallest vertebrate skeletons. Also, they reproduce very quickly so they can build up large populations in a hurry as the need arises, and the adults are relatively long-lived, living about five months and able to survive long periods without food, giving the colony resilience during slow collecting periods.
 
Also, they don’t like feathers, fur, or skin and most organs or dried blood. I have never seen a case of beetles from the dermestid colony attacking a bird or mammal skin collection. Usually skins are attacked by integument specialists that are much smaller. Since the species we have chosen don’t like skin, feathers or fur, they are not a real danger to these collections. I have found a D. maculatusinfestation in only one drawer in our entire skeleton collection, and that was when it was still housed in an unsealed wooden cabinet with no fumigant. This was a poor curatorial arrangement and the specimen, a large cervid, had not been thoroughly cleaned before being cataloged and installed in the collection. Even so, the bugs really can’t hurt anything in the skeleton collection, now can they?
 
I have heard reports of Dermestesattacking anthropology collections where they have specimens of thick, fatty hides or other, usually hide- or grain-based items. For the safety of all of the organic collections, care should be taken to make sure that beetles do not escape. The point should be made, however, that these beetles are of much less concern than other museum pests and normal museum curation of specimens should prevent any problems from being caused by a beetle colony.

 

THE PROPER DERMESTARIUM

As was pointed out earlier, D. maculatusworks well in museums because they cannot fly at room temperatures (below approximately 80ºF), therefore they are easy to contain in any type of container that they cannot crawl out of or chew through. We have had good success using old porcelain-lined steel appliance cabinets salvaged from the city dump, but most refrigerators and other appliances aren’t made with a porcelain liner any longer. An aquarium can be modified to hold bugs by trimming the silicon cement away on the inside corners so that they can’t climb out, but then the aquarium will never again hold water. We now use a plywood box lined with epoxy resin. This box was built so that we could fit elephant bones and antlered skulls inside and has worked so well that we have retired all other boxes. The Epoxy resin is much slicker than the porcelain we used previously and a bug has never crawled up its sides. It is also very hard and prevents the bugs from boring out through the bottom or sides as in ordinary wooden boxes.
 
A lid is necessary just in case the air-conditioner breaks or the custodian accidentally unplugs it with his mop and doesn’t notice, allowing thousands of bugs to spread throughout the building, not that anything like that has ever happened. More importantly, the lid keeps other insects out. There are many other species attracted to the smell of carrion which might cause problems in the utopian monoculture you are trying to maintain. Also, ventilation is very important to the health of the colony. A dermestid colony needs a large, unrestricted exchange of fresh air.

 

SUBSTRATE

When starting a colony you will need to provide a substrate where the bugs can shed their larval skins, pupate, lay eggs, and hide small bones they have stolen from your skeletons. In the past we always used number six Purina trout chow to start a new colony. I think you might just as well use puppy chow, gravy train, or mighty dog brand dog food, which is more easily obtained. It does have really big pellet sizes but this doesn’t seem to matter. It still provides a place for the bugs to pupate and so on and besides, it quickly becomes covered and mixed with an ever growing layer of frass, as does any substrate. In fact before long, the frass IS the substrate. We have found that the deeper the substrate, the bigger the colony, and therefore, the faster we can prepare skeletons. When the frass builds up to about four to six inches there seems to be no added benefit in allowing it to get any deeper. We have experimented with substrates almost a foot deep. Needless to say, there is less room for large specimens if you allow it to get too deep.
 
Specimens should be presented to the colony in a paper (gray board) tray covered with moist paper towels. The bugs need water and prefer it daily, although they can survive several days without water, especially if the humidity is over 50%. If a specimen is too large to be finished overnight, place a paper towel over it and dampen it with a spray bottle. The bottom of the box and the paper towel should dry out overnight. If it doesn’t you are getting them too wet.
 
To prevent the occurrence of museum mites, Lardoglyphus zacheri,be sure to keep the substrate perfectly dry. Mites have not been able to survive in our colonies so long as the substrate has been kept dry. This is the number one rule of bug colonies: Keep the substrate dry; keep the paper towels and specimens wet. Substrate DRY–specimen WET.
 
While paper towels are much preferred over cotton or other materials for use in the colony, be careful not to use paper towels from companies which treat their trees with juvenile growth hormones. Foresters sometimes use these chemicals to inhibit the larval growth and development of tree borers in their forests. Paper products made from treated trees may still contain these chemicals and if placed in the colony will inhibit dermestid growth. Corrugated cardboard may also be affected and should not be used as a substrate or for specimen trays in the colony. The bugs love to bore into the cardboard layers and hide inside. If a box is used in a colony it should be frozen for 72 hours to kill hiding bugs and prevent their escape into the museum building.
 
Care should also be taken to stir up the frass substrate as little as necessary. The larvae of Dermesteshave microscopic, barbed hairs that go airborne when the frass is disturbed. For this reason, the bug dust is hypo-allergenic and people with asthma, bronchitis or allergies should avoid working with the colonies.


FOOD PREPARATION

Before placing any specimen in the colony attach the specimen data on an etched metal tag or high rag content paper label attached with a thick cotton cord or stainless wire. Permanent carbon based ink should be used on paper tags to withstand immersion in water and ammonia. If the paper tag becomes soaked with blood or fat replace it so that it is not eaten by the bugs. A specimen whose data is lost is worthless.
 
The dermestids love fresh meat. Because they are usually found on dry carcasses in the wild they were often assumed to prefer flesh in a dried condition. Nothing could be further from the truth. They are apparently prevented from feeding on fresh meat by competition from the masses of fly maggots that quickly overwhelm most fresh carcasses, but when the carcass dries to a point where maggots cannot survive the dermestids move in. Since the dermestids don’t like skin, they may also be delayed until other carrion feeders have opened the hide.
 
Thus, the biology of the dermestid gives us the option of feeding specimens to the bugs in either a fresh or dried state. This is very useful. Small specimens that can be prepared overnight or in a relatively short period of time can be given to the bugs still fresh, immediately after skinning. You could also put really big things in the colony when they are fresh, but they will turn rancid and smell to high heaven before the bugs have a chance to eat all of the flesh. However, remember that it takes three or four days for the eggs, which are usually laid on the flesh of a fresh carcass, to hatch. Obviously if you want to maintain a colony that is large and vigorous, you need to feed it things that are big enough to provide places for egg laying which will not be eaten in less than three days.
 
For large specimens, drill holes in the long bones of the skeleton large enough for the largest larvae and adult beetles to enter and exit the marrow cavities. After removal from the bugs they must be fumigated, flushed out with high pressure hot water, and then dried before placing in the collection.
 
Ideally, large skulls should be available when you want the colony to grow or maintain high populations. A single fresh deer or bear skull will cause a population explosion of remarkable proportions. The downside is the putrid, gagging smell caused by a rotting brain of this size. But do not remove the brain unless you are an incredible pansy. The bugs love it, even when it begins to ooze out of the foramen magnum, and it provides a wonderful source of food and moisture. If you are NOT trying to increase the population of your colony, then the brains should be removed to reduce the smell and to increase the speed of specimen preparation.
 
Usually, we try to keep the smell down to tolerable levels by skinning, gutting, fleshing , and drying all specimens. By doing so, the specimens can actually be stored in a standard museum specimen-cabinet until needed to feed the colony. When they are to be placed in the colony they should first be soaked in hot tap water to kill any unwanted pests before introduction into the colony, and to soften the tissue so that it is more palatable and suitable for egg laying. Experience is needed to learn to recognize how wet a specimen should be before being presented to the colony. If it is too wet, the bugs will eat the flesh, but the bones will usually disarticulate and they will be covered with a thick paste of dried dermestid diarrhea. This does not harm the skeleton in any way, but extra preparator time will be needed to wash the skeleton. If it is too dry, the bugs will have difficulty eating the flesh, there will be little egg production, and it will have to be removed and resoaked, dramatically slowing down specimen preparation time.
 
Dermestids hate mummies. Remember when we said they don’t like skin, fur, or feathers? That’s the definition of a mummy. OK, actually the definition of a mummy in museum parlance is an animal that has died, been attacked by every carrion eater in the area, dried in the sun for months if not years, and then dragged into the museum by some biologist too lazy to collect a fresh one. Because they can see bones sticking out they think it is almost done. It will actually take longer to prepare than ten fresh carcasses of the same species. It must be soaked to soften the skin, then skinned, then washed and resoaked to remove all the dirt, sand, and maggot slime plastered all over it and then placed in the colony. They won’t eat the brick hard flesh until you have resoaked it around ten more times. Dermestids hate mummies.
 
There is one thing dermestids hate more than mummies, that is a specimen that has been stored in formaldehyde. Formaldehyde is a poison. You are trying to get the colony to eat poison. Today, most fluid specimens are fixed in formalin and stored in a buffered solution of 60% or 70% ethyl alcohol. If these specimens are soaked in water or beef bouillon for a few days the bugs will usually eat them. If you have a really old specimen that is still stored in formaldehyde it probably is not going to be met enthusiastically by the colony. If you have a really big colony you can sometimes get them to eat small amounts of this material by taking all specimens out for a couple of days to get them really starving, then stick in the formaldehyde soaked specimen. The specimen may be cleaned but the colony will be mostly killed off. No matter how many times you resoak it, it is still poison.
 
Another problem for dermestid colonies are small amphibians and small juveniles of any Order. These are not ossified or ankylosed sufficiently to prevent damage by a big beetle colony. To prevent damage, the specimen should be cleared and stained and kept far away from the bugroom. If someone insists on having such a skeleton eaten by the bugs, the specimen should be placed in a glass jar and a few hand picked bugs placed in the jar so that they can damage the specimen slowly. Actually, amazingly small specimens can be prepared by placing only a very few, very small larvae in the jar with the specimen and watching their progress very closely. These tiny bugs cannot eat very fast, so the specimen usually has to be resoaked to resoften the tissue so the tiny bugs can eat it. Resoaking in hot tap water will also kill any bugs hiding in the specimen and keep them from escaping. This usually will need to be repeated several times and is therefore very time consuming and costly.
 
Sometimes specimens which don’t dry quickly enough when being prepared for the bug colony become covered with mold. This is easily fixed by soaking in hot water and scrubbing off the mold with a stiff brush. The bugs will have no problem eating it after scrubbing. This problem is avoided by simply placing the specimen on a freezer shelf and allowing it to be dehydrated or freeze-dried by the freezing process. This is even faster in a frost-free freezer model which is usually avoided for storage of specimens to be prepared as skins. Specimens stored in freezers are also kept free of pests which might be introduced into the colony, but still must be soaked in hot water to thaw and rehydrate before placing in the colony.
 
In a nutshell, it is best to feed small things fresh, large things dry, and to feed large fresh skulls to build the population.

 

FINAL PREPARATION

When all of the flesh has been eaten by the bugs the specimen is ready for removal. In order to prevent bugs from escaping the bug colony and possibly attacking museum collections, the specimen must be fumigated to kill any bugs that may be hiding inside the bones. This can be done in several ways:
Immersion in hot water. This is excellent for small specimens and birds in particular but on large specimens the bones may crack if the water is too hot. Hot tap water is hot enough to kill the adults and larvae, is very quick and very safe. On large mammals it may crack the teeth if extremely hot water is used.
Immersion in Ammonium Hydroxide. This is even better than hot water but requires a really good fume hood.
Freezing. By placing the specimens in a house-hold type freezer at —18ºC to —20ºC for 72 hours all bugs should be killed. Large mammal skulls should not be frozen because this may also crack the teeth.
 
My particular favorite is hot water, which streamlines the process of cleaning specimens because it not only kills the bugs and is fast, but also helps to rinse off any bug-dust and frass clinging to the bones. Water is also user friendly. Combined with soap it can even be used for personal hygiene.
 
After killing the bugs the skeleton should be soaked in water for several hours to rehydrate any tissue that the bugs might have missed. Soaking makes such tissue swell approximately ten times its size when dry. This makes it easy to see and scrape away with a scalpel or knife of appropriate size. Absolutely all flesh and connective tissue must be removed from the bones.
 
Use fine pointed forceps to remove any dead bugs and larvae from inside the skull, auditory bullae, vertebrae and other small openings.
 
One advantage to killing bugs with hot water rather than with freezing or ammonia is that more bugs actually escape from the skeleton making it easier to clean. This can save a tremendous amount of preparator time. You can even rescue many of these bugs from the water surface if done quickly, before they are scalded, and return them to the colony.
 
There is a wide variety of opinion on degreasing specimens. In the UMMZ, for example, the fish people don’t care to have their specimens degreased. They say that is part of the fishy ambiance that they like in their work. On the other hand the bird division has an almost fetish desire to degrease their specimens. If you want to remove the oils from the bone there are several options here as well:
Ammonium hydroxide. To me this is the tool of choice. It does not bleach the bone and change its natural color, but does a good job of removing the oils. It will decalcify the bone, so bones cannot be left in it for long periods. Small specimens should be soaked overnight, rinsed then soaked in water for an equal period of time to remove the ammonium hydroxide. Large specimens can be soaked for longer periods, even weeks, the exact time a matter of experience, judged from the size of the specimen. It actually takes a long time to damage a specimen, so undue concern is not warranted. (However, I once found a squirrel skeleton in a jar of ammonium hydroxide that had long been forgotten on a shelf. The skeleton, although completely intact, was like a rubber toy!) Its only other drawback, which can be a large one, is the need for a fumehood. It can be unpleasant and even dangerous without the proper equipment to clear its fumes.
Hydrogen Peroxide. Can be used to degrease and also bleach the bones. It will make them a very bright white and remove all of the oil, but will also decalcify them if soaked too long. The same cautions apply here as above. Peroxide has the advantage of being readily available to the non-museum user and also being user friendly with absolutely medicinal qualities.
Diaper soap. A mild soap, not a detergent with perfume or other additives, can be used to simmer the skeletons and remove the fats and oils. This has the disadvantage of being very, very slow and labor intensive. If only a single or few specimens are to be prepared and you don’t want to bleach the skeleton, this works very well.
 
Once the specimen has been cleaned and degreased, it should be dried and placed in a clean box that has not been used in the bug colony and refumigated before installation in the museum collection. With practice and experience your dermestids can be used to prepare large numbers of specimens very quickly all-year-round.