Diachasmimorpha juglandis is a parasitoid wasp in the family Braconidae. Like other braconids, it is relatively large; while most parasitic wasps are probably less than 5 mm long, D. juglandis can get nearly up to 1 cm.

This species is categorized as a "solitary larval-pupal endoparasitoid," meaning that an egg is laid inside the host - in this case a larva (or close relative) of the walnut fly Rhagoletis juglandis, and a single adult wasp emerges from the pupal stage of the fly, having devoured it from within.


D. juglandis females, shown in these pictures, have to find their hosts without ever making visual or physical contact with them, because they are feeding within the husks of walnut fruits. Once a wasp has landed on the fruit, she walks around its surface, pausing every few seconds to feel for vibrations beneath her, caused by moving fly larvae. When she has located one, she inserts her long ovipositor through the husk (above right), and hopefully, into the body of the fly larva, where she lays a single egg.

The tiny wasp soon hatches, and bides its time as a first stage larva, cruising through the bodily fluids of its host, sustaining itself on stored fat. Interestingly, the length of the first stage is variable, depending on what stage the fly was at when parasitized; the wasp needs to wait for the fly to begin its pupation (which it does after dropping out of the walnut and digging down into the soil). At this point, the wasp molts into the next stage and begins devouring the entire body of the fly, a task made especially easy because the fly tissues have begun breaking down on their own in preparation for reassembling as an adult fly.

Notice that in the picture above and at the left, of a recently hatched wasp larva, it has a rather large sclerotized head, somewhat like a helmet with jaws. This structure is present only in the first stage; after the first molt, the wasp larva looks like an amorphous white blob.

The reason for the sclerotized head and jaws has to do with the fact that these wasps are solitary - it is only possible for one wasp to emerge from a single host. It is possible for two wasps parasitize to the same host, so that there are two first-stage larvae present at the same time. Because the host only provides enough food for one, these wasp larvae must fight to the death within the host - which all the while continues on its merry way in the walnut, eating fruit until it is full grown and ready to pupate, oblivious to the battles occuring within its body. The jaws on the wasp allow it to easily attack and kill a rival wasp in the fly.

Which wasp wins? This is a fascinating question that has been studied in several solitary parasitic wasp species. In most cases, the one that got there first has the advantage, because it has had time to feed and grow a bit, making it stronger than any subsequent intruder. But back in the early part of the last century, some biologists studying these interactions in a related Diachasmimorpha species in Hawaii (imported there for biological control of medfly and oriental fruit fly) came to a fascinating conclusion following their dissection of hundreds of parasitized flies. They found that they could distinguish how old a first-stage wasp larva was by how fat it was - newly hatched larvae were skinny, but a wasp that had been in the host for a few days had begun to get tanked up on all the fat it was feeding on in the host. When they found two wasps in a death struggle, the skinny one had killed the fat one.

So apparently, when a wasp first hatches, it swims around the host before it tanks up, perhaps looking for any other wasp larvae. When it encounters a fat larva that has been there alone for a few days, it is an easy task for the skinny and much more maneuverable wasp to pop the older one with its jaws like a balloon (upper inset on photo above right).

By the time the fly begins to pupate, it is in the soil and will not be parasitized by another wasp; at this point there is only one wasp survivor inside of it. The sclerotized head of the first stage wasp larva is no longer necessary, and when it molts to the second instar upon fly pupation, the helmet is gone.

Both the host, and thus by necessity, the parasitoid, have just one generation a year. There are a few short weeks as adults for them when the flies emerge from the soil and oviposit in ripe walnut fruits, and then the emerging adult wasps try to find them and parasitize as many as possible. Flies and wasps then diapause (a type of insect hibernation) in the soil for nearly a year until ripe walnut fruits are available again.

Labels: cool bugs, ecology, flies, insects, wasps

I thought about doing a post on all the wasp mimics out there, but within the flies (Diptera) there are plenty, and it clearly evolved multiple times - in most cases, not all the species within the following family are mimics. Obviously it would be some benefit for any insect to be thought a wasp by a vertebrate predator. Flies cannot sting for defense, so some of them just look a lot like wasps so predators will think they can sting. The ways in which they mimic wasps are fascinating.

The following families include wasp mimics: Micropezidae, Conopidae, Mydidae and Syrphidae. I'm surely missing some - don't be shy about pointing it out, all you Dipterists out there.

There is a whole family of bee mimics as well, the Bombylidae (the bumblebee genus is Bombus). They are big fuzzy things (below right), but if you look closely, you will see only two wings, which gives away their lineage - all bees and wasps (and all orders of insects except for the flies) have four wings.

But I'm more interested in the wasp mimics here. I'll start with my favorite, a Micropezid I caught in Costa Rica, at the La Selva research station. These are fantastic mimics, and a still photo just doesn't do them justice because their behavior is an important part of the package. You can see the fly has a pointy abdomen, which helps, and when grabbed, it pokes its abdomen into the grabber's skin repeatedly as if to sting. (Kinda cute, since it's completely harmless.) The other important combination of morphology and behavior has to do with the long forelegs, which end in white tips (which you should be able to see in the photo, along the edge of my thumbnail). In the tropics especially, the long antennae of stinging wasps have white or yellow tips. Flies, as a group, have very small antennae, but this family of flies has long legs. It was a quicker evolutionary step for the mimic species to use its forelegs to mimic antennae, than to develop long antennae itself. So you will see this fly walking rapidly along leaves in the manner of wasps, tapping its forelegs in front of it just as wasps use their antennae. It's really amazing to watch. (Although this fly family is more ubiquitous in the tropics, there are North American species and I have seen them in central Virginia.)

Conopids have a generally different look, mimicking thread-waisted wasps (Sphecidae) rather specifically. A common wasp-mimic morphology is to have a somewhat constricted abdomen, because a distinguishing character of the Hymenoptera (ants, bees, wasps) is a distinct constriction in the first few abdomenal segments, which means that hymenopterans are more or less restricted to liquefied foods, but also allows flexible reach for the abdomen when stinging prey or for defense. The conopids combine this with the elongated abdomen characteristic of sphecid (digger) wasps. I'm not aware of any specific behaviors that help promote their ruse.

Some Mydidae (mydas flies) apparently go for the pompilid (spider wasp) look. According to the source for this photo of Mydas clavatus, Tom Murray, it is mimicking spider wasps in a particular genus, Anoplius. Pompilids have a quite characteristic look of a black body and darkly pigmented wings. The photo on the right is Anoplius.

The syrphids (hoverflies) are not so precise in their mimicry. Here are two, with one clearly mimicking a bumble bee, and the other just looking generally wasp-like with its black and yellow markings. Their behavior does not necessarily contribute to the show; as their common name suggests, syrphids spend a lot of time hovering, which is generally unwasplike.








Thus mimicry takes many forms. It is interesting that some mimics seem to be modeling specific insects while others just seem to have the general look of wasps or bees. Does the selection pressure differ for these mimics, and why? Perhaps the generalist mimics live where there are a big enough variety of stinging Hymenoptera that they don't need to get specific. Why do some converge on specific families? Is there a dominant model present in those habitats? I'll admit up front that I have not done a literature search, so I don't know what is known specifically about the evolution of mimicry in these groups. I just like them because they are so cool.

The only picture of mine above is the worst one by far, of the micropezid. The syrphids and Anoplius come from Forestry Images, a wonderful image database, and the rest are by Tom Murray, and used with his permission. See many wonderful fly images of his here.

Labels: bees, biodiversity, cool bugs, ecology, evolution, flies, insects, wasps

What do you think this sound is? Continue reading for the answer...

Since it's been two weeks since my last "cool bug" post, I thought I had better change the name of the series... we'll see where it goes from here.

Today's subject is a fruit fly, Rhagoletis juglandis. This is not related to the fruit fly of genetics fame, Drosophila melanogaster, which is in a different family. Nearly all drosophilids only eat fruit once it is rotting; flies in the family Tephritidae, including the genus Rhagoletis, feed on ripe fruit and thus are known to entomologists as the "true" fruit flies.



I will admit up front that these flies are mainly of interest to me as larvae (at the left), because they serve as hosts for one of my favorite parasitic wasps, Diachasmimorpha juglandis, below. R. juglandis larvae feed on and live in the fruit of the Arizona walnut (Julglans major) (i.e., the husk surrounding the actual nut), and D. juglandis females parasitize them through the walnut fruit skin.


The fly larvae live in groups in the walnut husk, sometimes by the dozens. All the larvae in a fruit may or may not have the same parents, if there have been multiple ovipositions in the fruit.




In the picture to the right is a mating pair of R. juglandis adults on a plastic walnut model. Males and females mate multiply, with several individuals if given the opportunity.





There are territorial contests by the males on the ripe walnuts while they are still hanging in the tree. This behavior is known as "boxing." The males stand on their hind legs and bat their forelegs and wings together. (In the picture to the left, the wings are only a blur.) The idea is that the winners of these contests have access to more females, who will come to the walnut to mate and lay eggs. Some poor females are forced to mate as they extrude their ovipositors to dig a hole in the husk in which to lay eggs; the males will grab them from behind and mate with them before they have a chance to oviposit. Sometimes, though, males are so intent on fighting with each other that they don't seem to notice a third male that is mating with the female on the fruit while they are going after each other.


While males are duking it out, mated females also get the opportunity to finally oviposit without harassment (left). A female drills a hole in the husk with the tip of her ovipositor (which eventually shows signs of wear) and deposits several eggs in a cavity just beneath the surface of the husk. These grow and feed inside the husk until they are ready to pupate, when they exit the fruit and burrow into the soil. Sometimes there are so many larvae within the husk of a walnut that their feeding is audible. Click here to listen to the sounds of feeding fly larvae in a walnut.

Unfortunately for the larvae, the racket they make chowing down on the walnut is their undoing... as will be revealed in the next Cool Bug of the Fortnight!

Here are references for more information on Rhagoletis juglandis:

Papaj, D.R., 1994. OVIPOSITION SITE GUARDING BY MALE WALNUT FLIES AND ITS POSSIBLE CONSEQUENCES FOR MATING SUCCESS. BEHAVIORAL ECOLOGY AND SOCIOBIOLOGY 34 (3): 187-195.

Henneman, M.L. and Papaj, D.R., 1999. Role of host fruit color in the behavior of Rhagoletis juglandis (Diptera: Tephritidae). Entomologia Experimentalis et Applicata 93:247-256.

Nufio CR, Papaj DR, Alonso-Pimentel H, 2000. Host utilization by the walnut fly, Rhagoletis juglandis (Diptera : Tephritidae). ENVIRONMENTAL ENTOMOLOGY 29 (5): 994-1001.

Labels: biodiversity, cool bugs, ecology, evolution, flies, insects

We have two male black and white cats (also known as tuxedo cats, or holsteins), shown here. The older one, Ippy, is well known in the neighborhood for playing with kids in the park a block away, and following people walking by our house to the grocery store across the park, waiting for them outside, and walking with them back to our house. Ippy was almost a year old when he adopted the younger, Tacaribe, without involving us in the decision (Tacaribe had two cameos on Tigerhawk soon after Ippy adopted him).

We had already discussed the interesting behavior of the black-and-white males with our vet, who told us that every single one she had run across had a very playful, mischievous and outgoing personality. She thinks it is much more pronounced in the males than the females. My husband's family had a black-and-white that was very similar to Ippy in his personality. He was people-centered rather than place-centered, which is uncommon in cats (a solitary species). For example, when they traveled and stopped in hotel for the night, they could put him out and he would come back to their room in the morning. We have come to the conclusion that the trait involved with such behavior is probably one that causes neoteny, or retention of juvenile (kitten) behavior in the adult.

This is not a particular breed of cat; Ippy and Taca have very different body types, and really only are similar in their coloring and spunky behavior. Looking in a big cat book at a book store once I noticed that many listed breeds had a black-and-white form, generally distinguished by a mostly black cat with white feet, chest/belly, and often forehead spot. For some reason, this behavior seems to be associated with this particular black-and-white coloring. Although this might seem a strange association, it is not unheard of. In Drosophila, the yellow gene, which is involved with the production of melanin (pigmentation) in the flies, also has neurological effects. Mutant males without the normal yellow gene have lower mating success than normal males, and there are apparently effects on larval foraging as well.

Behavioral neoteny in flies wouldn't really make much sense, because flies undergo complete metamorphosis - there wouldn't really be a way for an adult to act like a larva. But it is intriguing that there seems to be a similar connection between pigmentation and behavior in my cats.

Labels: behavior, cats, flies, genetics