Midges of the Macabre

In a recent post, I commented that derived members of an insect 'order' could sometimes be all but unrecognisable as belonging to that order. Take a look at this:

Miastor sp., copyright Charles Olsen.

Believe it or not, sitting in the middle of that photo is a fully reproductively mature insect (I'm not so sure about the maturity of the other individuals). In fact, it's a fully mature fly. Miastor is a genus of midges found living in rotting wood or fungal fruiting bodies. Members of this genus are found worldwide; I believe that several species are recognised, but distinguishing the individual species is extremely difficult. Miastor exhibit what is called paedogenesis: they can become reproductively mature while still in the larval state. They are not the only genus of the family Cecidomyiidae to exhibit this process; another such genus, Oligarces, is found in similar habitats. Miastor is probably the best known such genus, in part because it has been cultured for study in the laboratory, and in part because of the decidedly macabre way in which its paedogenesis plays out.

Each paedogenetic Miastor larva develops several eggs within its ovaries (generally four to ten or more—Harris 1923). No males are involved in this process: the larvae are parthenogenetic as well as paedogenetic. These eggs hatch while still inside their mother, after which the daughter larvae are nourished by the mother's own tissues. Eventually, the daughters are not born so much as they escape. In the words of Quammen (1985), "While the food lasts, while opportunity endures, no Miastor female can live to adulthood without dying of motherhood". But karma still seems to have its place, because by the time the daughter larvae escape they carry their own fate within them: they emerge with their own eggs already developing inside them.

Metamorphosed male Miastor metraloas, copyright John Plakidas.

In this way, a Miastor colony can go through an entire generation in as little time as two weeks, until changing conditions (such as exhaustion of food supplies, or a change in season) induce a change in tack. Larvae are produced that do not reproduce paedogenetically in the way that their mothers did, but instead pupate in the usual way to emerge as more ordinarily formed midges, both males and females. As with another paedogenetic insect that has already been featured on this site, the beetle Micromalthus debilis, these metamorphosing larvae can be readily distinguished from paedogenetic larvae. Not only do they not produce the precocious reproductive organs of their fellows, but they have visible imaginal discs (the clumps of tissue that develop into adult structures during metamorphosis), and have eyes that are clearly separated on top rather than touching as in paedogenetic individuals (Harris 1923). After the mature adult midges emerge from their pupae, they can disperse in search of new habitats, seeking food for their offspring and mates for themselves in order to begin the cycle again.

REFERENCES

Harris, R. G. 1923. Occurrence, life-cycle, and maintenance under artificial conditions, of Miastor. Psyche 30 (3–4): 95–101.

Quammen, D. 1985. Natural Acts. Schocken Books.

Juniper Gall Midges

Sometimes, the evidence of an insect's presence may be much more visible than the insect itself. Imagine passing by a common juniper tree Juniperus communis and seeing a structure like the one in the photo above (copyright Jean-Yves Baugnée). You might think it was some sort of reproductive structure. You would be right, though it is not the tree that is reproducing. This is the gall of a juniper gall midge Oligotrophus juniperinus, and were you to cut the gall in half you would possibly find a single gall midge larva lurking within. Many insects (and other animals) cause the development of galls on their host plants, thus providing themselves with both shelter and food in one convenient location.

Male Oligotrophus betheli, from Simova-Tošić et al. (2010).

The adult gall midge is a minute, very delicate fly, unlikely to be spotted by the casual observer. Gall midges are classified in the family Cecidomyiidae, an extremely diverse group of which not all members cause galls as larvae (some feed on plants without causing galls, others feed on fungi, a few are even predators or parasitoids). Cecidomyiids are divided between a number of subfamilies and tribes, with Oligotrophus belonging to the tribe Oligotrophini. In the past, this tribe has been used to cover a heterogeneous mix of relatively unspecialised cecidomyiids, but the most recent classification of the tribe strips it down to two genera, Oligotrophus and Walshomyia, found in the Holarctic region (Harris et al. 2006). Adults of these genera have legs with simple tarsal claws and long empodia (the soft pads between the claws), and as larvae they all live in galls on trees of the cypress family Cupressaceae. The exact form of the gall produced may differ between species, and it is often (though not always) possible to determine the species responsible for a gall by its form. For instance, three species that cause galls on Juniperus communis in Europe are Oligotrophus juniperinus, O. panteli and O. gemmarum. The first two species have galls formed from whorls of leaves pressed into a vase shape, but whereas in galls of O. juniperinus the leaves splay outwards towards the tip, in galls of O. panteli they remain parallel. The third species, O. gemmarum, has much smaller galls formed from only slightly modified buds; though very different from mature galls of the other two species, they may be confused with young undeveloped galls (Harris et al. 2006).

Despite their diversity, and the fact that some species are economically significant to humans, cecidomyiids are not a widely studied group. Part of the reason for this is that their small size and build makes them difficult to handle; diagnostic work on adults often requires slide-mounting them. I have made one not-very-successful attempt at slide-mounting cecidomyiids, and I can confirm that it is a fiddly process. Because the different body parts often have to be examined from different angles, slide-mounting first requires dissection of the animal into sections (so, for instance, the head can be placed on the slide face-on, the body side-on, and the terminalia top-up). In my experience, instructions for slide-mounting animals requiring such dissections will always tell you to arrange the various bits appropriately on the cover-slip before placing the slide (or the other way around, if you prefer). And if you know how to attach slide to cover-slip without having all your carefully arranged body parts immediately zooming off to a completely different spot on the slide from where you put them, then you're a far more skillful slide preparer than I am.

REFERENCES

Harris, K. M., S. Sato, N. Uechi & J. Yukawa. 2006. Redefinition of Oligotrophus (Diptera: Cecidomyiidae) based on morphological and molecular attributes of species from galls on Juniperus (Cupressaceae) in Britain and Japan. Entomological Science 9: 411–421.

Simova-Tošić, D., D. Graora, R. Spasić & D. Smiljanić. 2010. Oligotrophus betheli Felt (Diptera: Cecidomyiidae), a new species in the fauna of Europe. Arch. Biol. Sci. 62 (4): 1219–1221.