Flies may be my least favourite insects, but there’s no denying that they are extremely diverse and of special economic importance. This series will be similar to my beetles series, but going into more details, except where phylogeny is concerned. Systematics is, as always, under constant revision. The families I’ll be introducing in the series are all well accepted, I will just not be ordering them into any phylogenetic system.
Before getting to any of that though, let’s look at the morphology of flies.
As all insects, a fly is divided into three parts: the head, thorax and abdomen.
The head is, as in all arthropods, specialised and unique. The most striking feature of a fly’s head is the two enormous compound eyes (although keep in mind that they may be reduced in some taxa!). Each eye consists of thousands of facets, which are the corneas of the ommatidia (the individual eye cells, if you will). Facets may vary in size and are not uniform; this is best seen in the colour of the eyes – some flies even have patterned eyes. In some flies, they eyes almost meet – these are referred to as holoptic; when they are widely separated, they are dichoptic. There is no pattern to be found here: in some taxa, this is how sexual dimorphism is expressed, while it is a diagnostic character for some families. In addition to the compound eye, flies usually have one or more ocelli (some taxa have none).
The vertex, frons, face and clypeus are pretty standard (the face is special for the Diptera, but there’s not that much to say about it), so we’ll jump straight to the antennae. They are extremely variable, which is both a blessing (for the taxonomist) and a curse (for the phylogeneticist). The standard antenna consists of a scape, pedicel and a flagellum consisting of a variable number of flagellomeres. The scape is the attachment point to the head and the pedicel detects the movement of the flagellum (through Johnson’s organ, a mass of neuron cells in it). It’s the flagellum that is always varied and can’t really be generalised. In the fly above, there are only 2 flagellomeres: the first one is grossly enlarged, while the second one is just a hair on it (called the arista). Other flies have regular-looking antennae, though!
The mouthparts of flies are similarly variable, but can at least be categorised according to feeding type. We will look at the individual families as we get to them, and we will not discuss the evolutionary derivation of the mouthparts. In flies, the whole mouthpart system found in other insects is tightly-integrated to form a single tubular organ, the proboscis. The ‘walls’ of the proboscis are formed by the labrum, labium and hypopharynx. Within the labium (it forms a ‘valley’) lie the paired mandibles (in most cases only functional in females) and maxillae. The labrum is attached to the clypeus and thereby forms the top of the food canal. The bottom of the food canal is the hypopharynx, which also contains the salivary canal. There are two feeding types: the piercing and sucking type (think midges, mosquitoes) and the licking type (most flies). There is too much variation in these to discuss them properly; for example, mosquitoes (piercing and sucking) use all mouthparts to pierce the skin (see picture above), while dolichopodids only use their toothed labrum as a knife. In non-biting flies, saliva is used to soften food, which is then licked up.
Now we come to the thorax. As you can see in the picture above, there’s not much to it – the prototypical insect has a pro-, meso- and metathorax. In dipterans, the mesothorax is greatly enlarged, and the other two vestigial; hence why only the mesothorax is meant when referring to the dipteran thorax. The prothorax is still distinguishable, as it is where the head is attached (via the cervix). The enlargement of the thorax is meant to accomodate the flight musculature of the single wing pair.
Of course, it is that single wing pair which is the most famous characteristic of the Diptera (it’s what they’re named for, after all). The second pair of wings is modified to a pair of halteres. The halteres consist of the base, stem and knob (fancy names: scabellum, pedicel, capitulum) and serve as stabilisation organs during flight (as testified to by the unusually high number of sense organs in the base of the haltere; see picture above).
As I stressed in a few places before, I hesitate to go into wing venation issues due to the constant danger of using an outdated naming system for the wing – just keep in mind that the wing venation is very important and is still the best way to distinguish insects. The picture above shows the ground plan for the dipteran wing. The different colours show different categories of vein; the areas between them are called cells and also have names (derived from the vein). The reason I don’t want to go into the details is because some of those veins may be seen by other authors as belonging to another category, and there’s no need to get into any controversies.
The other group of appendages are the legs. As in all insects, each leg is made up of five segments: the coxa, trochanter, femur, tibia and tarsus. There are three pairs of legs, and each segment of each pair has some structure to distinguish it from the rest. The coxa connects the leg to the thorax, articulated through the coxifer. The femora and tibiae are almost always the longest leg segments and usually have various forms of armaments (spurs, etc). The tarsi comprise five tarsomeres, all of which are loaded with mechano- and chemoreceptors and are thus quite fun to look at under an electron microscope. What this also means is that the legs of flies aren’t just locomotory appendages; they can be used for tasting and sensing the environment; some groups have modified them to be raptorial appendages or as antennae; they are ubiquitously used as cleaning organs due to their spurs and hairs; sexual and competitive selection has also made its mark on the legs, as they are often colourful (either as predator warnings or to impress mates).
Finally, we come to the abdomen. It retains the primitive state of having 11 segments, with the terminal 11th segment (proctiger) bearing a pair of cerci and the anus. The copulatory organs are also found on the abdomen. The bagina is located between segments 8 and 9, the penis (aedeagus) between segments 9 and 10. An entomology textbook would now go into details of how the genitals look like, as they are often the only way of differentiating species. I will just outline the basic structure of them.
The female genital opening (vulva) opens into a genital chamber (vagina). There is one fused oviduct connected dorsally to the vagina via the primary gonopore. Spermathecae are present, in which sperm is stored and released on eggs as they are released from the oviduct; they are open to the outside and are where the penis deposits the sperm. Finally, there are accessory glands that produce chemicals that aid in egg laying (adhesives to glue eggs to a substrate or to themselves, for example).
The male terminalia are more complex, with several parts getting coopted in some cases to form penis guards or sheaths – there’s no point in discussing these here. Instead, we’ll just look at the aedeagus (penis), whose role is obviously to transfer sperm to the female spermathecae. It’s incredibly variable (hence why it’s so popular in taxonomy). Some families have a simple membranous aedeagus, as they only produce a spermatophore. Those families that don’t produce one often have a sclerotised ventral guard for ease of penetration (it’s okay if you’re imagining this stuff in your head. I hope.). In addition to the aedeagus, it’s worth mentioning the sperm ducts, which (only in non-spermatophore producing species) are sperm pumps (a sperm sac, contracted by muscles, pushes sperm up a vas deferens into the sperm ducts, which are connected to the aedeagus).
A specialty that male dipterans have is the ability to rotate their terminalia, as well as flex their abdomen to allow for a wide variety of sexual positions, as can be seen above.
The result of mating is a bunch of fertilised eggs, out of which come the larvae. These are so variable that there is virtually no way of certainly distinguishing dipteran larvae from those of other insect orders. The only thing they all share in common is that they are all apodous (have no legs) and that they actively move with a sense of direction (many apodous larvae seem rather lost when they move and don’t seem to know where they want to go). Thereofre, I think it’s rather useless to try and give a general overview of dipteran larvae and will leave it for the individual families.
Original Picture Sources:
Grimaldi & Engel’s Evolution of the Insects.
Sex picture from the Manual of Palaearctic Diptera.
The wing colour pic is my own creation. But based on facts!