When you first look at maggots they all look the same – white, squashy and featureless.  However, with more careful examination you can find some distinct features, at least in older maggots.

There are three stages of maggot development after an egg hatches.  The first stage, or instar, is almost featureless and it is virtually impossible to identify them.  The second instar has more features, but still not easy for identification, and it is the final or third instar that has the most distinctive features.

Most maggots are roughly cylindrical in body shape, coming to a point at the head end and more-or-less squared off at the tail end.  Below are several different body shapes:

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The head contains a head skeleton, which usually consists of a pair of mouth-hooks, followed by an intermediate piece (called a sclerite), and then a pharyngeal sclerite.

The mouth-hooks drag food into the maggot’s mouth.  Maggots that feed on soft materials like dung and other decaying material have flattened hooks like small shovels that pull the material in.  Carnivorous maggots that feed on carcases have hooks like knives that can slice meat off, and maggots that feed on fresh plant material have hooks with tiny saw edges to cut off pieces of plant material.

But the most spectacular are the hooks of maggots that are predacious on other maggots.  They have hooks that are down-curved and extremely sharp.  These hooks are held inside the predator’s head until it finds some prey.  Then it strikes at the prey and the hooks come out and slash down into the victim.  Two other side-pieces of the head skeleton than move outwards to lock the predator’s head in position inside the prey, and the predator then secretes digestive enzymes into its victim and feeds on it.  There’s no escape!

There is also a feature on the pharyngeal sclerite that can help towards indicating the diet of the maggot.  Many decaying substances that maggots live in, such as fresh cow dung, contain a lot of water which is not nutritious, so the maggot concentrates what it takes in.  The Figure above indicates the pharyngeal ridges that are found on the underside of the pharyngeal sclerite of such larvae, and they are a filter to remove water.  The maggot closes off its intestine just behind the pharyngeal sclerite, then swallows a dollop of dung and holds it inside the chamber of the pharyngeal sclerite.  It then closes its mouth, and squeezes the food blob against the ridges.  The watery part of the food goes down through the ridges into a lower chamber, while the food solids remain held by the ridges.  The mouth then opens again and the watery part is expelled back through the mouth.  Finally the valve closing the intestine is opened, and the now-concentrated food is swallowed.  It is a very neat system.

Flies with carnivorous habits don’t need to concentrate their food because it is protein rich, so they have no pharyngeal ridges, and parasitic flies likewise have no ridges in their pharyngeal sclerite because they feed on the concentrated body fluids of their prey.

Below are illustrations of the different types of head skeleton.  Fig 51.133 is from a soft food feeder; 12.169 and 168 are carnivorous; 51.142 to 145 are predacious; 6.40 is plant-feeding:

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The maggot’s body also contains two long breathing tubes or tracheae, one on each side.  Each trachea opens at the head end through an anterior spiracle and at the tail end through a much larger posterior spiracle.

The anterior spiracles are usually rather insignificant.  Each has a few small, lobe-like structures at its opening, though in a few species of maggot these lobes remain closed altogether.  This is not surprising since the maggot’s head is most of the time immersed in gooey material that would clog up the openings.  These are several different patterns of anterior spiracle:

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The posterior spiracles are larger and more obvious, mounted on the rear end of the maggot.  Each is usually a flat plate with several slits in it through which air can enter and carbon dioxide can be expelled.  Many species have three slits on each spiracle, and the shape and positioning of the slits can be very significant in determining which species the maggot is.

Other maggots may have more than three slits, or they may have an assortment of small round pores instead.  One or two have a spike so that they can probe into the air through clogging material, and in the case of a few maggots that are parasites of other insects, they stick their spike into the breathing tubes of their hosts to get air.

The plate of the spiracle also has a small round aperture beside the slits, called the button.  It is the scar left from the moult of the previous maggot instar.  In addition there are usually several small holes in the plate, and there may be rather flowery, branched filaments coming out of each hole.  These holes are the perispiracular glands, and they secrete a water-repellent fluid which helps the spiracle to clear immediately when it comes out of wet dung or whatever it was previously buried in.  After all, you wouldn’t get much gaseous exchange in and out of the spiracles if they were clogged up with goo.

The illustrations below show some of the different forms of posterior spiracles:

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So what do you look for when trying to identify a maggot?  The features of the head skeleton tell you mainly what the diet of the maggot is, though that of course cuts out any species that have different diets.  The most useful features are the hind spiracles – whether it has slits or holes or whatever, the number of slits and the pattern in which they are placed on the plate, and the position of the button.

However, there is also another feature that can be important, which is spines on the body.  Because maggots have smooth skins or cuticles, they would not be able to get traction to move unless they had something for gripping the surface.  So each segment of the body has one or more rows of small, sharp spines that can grip the surface on which the maggot moves.  And usually there are some spines that point backwards for forward movement, but also some that point forwards so that the maggot can quickly go into reverse, either to back out of goo or to escape a predator.  The lengths of the spines, the number of rows of them and their pattern of distribution can be very significant for identification.  Some species, especially ones that are parasites in mammals or birds, have many rows of quite ferocious-looking spines and can be quite spectacular.

One other body feature that can be of some use is the anal plate.  This is a plate on the underside of the larva where the anus opens, and the size and shape of the plate can sometimes be of help in confirming an identification.

Apart from the three stages of the maggots, there are also eggs and pupae in the fly’s life cycle.  Fly eggs are almost impossible to identify from their appearance, though some may have extra horns or filaments at one end.  You can also narrow down the possibilities from where the egg is laid.  Obviously an egg collected from dung is a dung-breeding species, from carrion would be some sort of blowfly, and an egg glued to the lower part of a horse’s leg will be a horse bot-fly.  The figures below show several different forms of fly eggs:

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The pupa is the stage after the three maggot stages.  The final (third) stage larva becomes quiescent after it is fully fed, and eventually the outer cuticle of the larva becomes hard, tanned and rounded, and inside this shell, or puparium, the remaining parts of the larva turn into an adult fly.  When the fly is fully formed the puparium splits open at the anterior end and the fly emerges.  It sits quietly for several hours while it inflates its wings and its cuticle hardens off; then finally it flies away.  Because the puparium is formed from the old skin of the maggot, it retains some of the features of the maggot.  The anterior spiracles are not usually discernible, but the posterior spiracular plates show up clearly and can be very useful in identifying the species.  The head skeleton is also retained, but inside the puparium so that you can only find it by opening up the puparium.

Below are several different types of puparium appearance.  The puparium in Figure 2.1118 has split open where the adult fly has emerged.

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All the Figures above are from Part 2 of Ferrar, P., 1987, A Guide to the Breeding Habits and Immature Stages of Diptera Cyclorrhapha.  Entomonograph 8, 479-907, Scandinavian Science Press.

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