While out walking recently, I noticed a dead caterpillar. Around it were the barrel-shaped pupal cases of tachinid flies, a large family of flies whose larvae feed internally on caterpillars.
Most tachinids, of which there are about 8,200 species, are ordinary-looking flies. With their colors of black, grey, and brown, they can be mistaken at first sight for houseflies or some kind of blowfly. They are parasites, getting their nutrition from their hosts.
Some species are long and narrow with long legs, but in others, the abdomen is much broader and often reddish at the sides. All tachinids are rather bristly and rest with their wings half open. The females, when looking for hosts, have a busy, searching flight which is distinctive.
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How Do They Feed?
As with ichneumon wasps, the larvae of the Tachinidae obtain their food from the body tissues of their hosts which are mainly moth and butterfly caterpillars, but also sawfly and beetle larvae and nymphs and adults of the Hemiptera.
When the larvae are fully grown within their host, they either change to pupae in barrel-shaped pupal cases within the dead host’s skin or bore out and form their pupae and pupal cases in the soil. The brown pupal case, in common with all higher ‘flies, is the hardened larval skin which forms a case for the pupa within.
Finding The Correct Host
While ichneumons seek out their hosts and lay eggs either in or on the hosts’ body, using their sharp ovipositor, tachinid larvae, which are all internal parasites, find their way into their hosts in a variety of different ways. This is partly because the females do not normally have a sharp ovipositor.
In the easiest method, the females of such species as Phryxe vulgaris and Phryxe nemea search out hosts and stick their slightly flattened eggs to the hosts’ skin. The flies’ eggs sometimes hatch immediately, or there may be a delay, however, the newly hatched larvae rapidly bores through the skin and into the hosts’ bodies.
The hosts sometimes try to shake off the parasitic larvae, but usually, they are indifferent. The eggs often fall off, though, or may come away with cast skins.
There are other methods employed as well. For example, Compsilura concinna, has developed a sharp ovipositor or a special spine with which to pierce the host’s skin so that eggs or larvae can be directly inserted within the host’s body.
Some female tachinid flies retain their eggs in a special body chamber until they hatch and can lay active larvae, with females laying between 100 and 200 eggs or larvae. If too many larvae invade a host, some may die. Most are solitary parasites, however, but up to half a dozen in one host is known to occur.
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What appears to be a haphazard method of host location is that used by numerous species that glue vast numbers of tiny dark eggs to the leaves of plants used as food by suitable hosts. Each female lays several thousand eggs, between 2000-3000, although this is sometimes higher.
These eggs can hatch for six weeks, but they do not do so until they are swallowed with the foodplant by the host, usually a caterpillar or sawfly larva. The fly larvae then hatch within the caterpillar’s intestines before migrating to other parts of the host’s body.
Another group of tachinid flies, including Tachina grossa, one of our bulkiest species, similarly lay small eggs in the neighborhood of suitable hosts. These hatch immediately into tiny migratory larvae, which actively search for hosts into which they quickly bore. The females of these species lay between 400 and 1000 eggs, and the larvae are frequently armored to prevent desiccation.
Most tachinid fly larvae parasitize butterfly and moth caterpillars by feeding on their body tissues. They do not, however, inflict fatal damage until late in their larval life. The larvae breathe through spiracles situated at their rear, using the hooked mouthparts or raspers at the other end to tear off chunks of the host’s tissue.
Although these host-locating methods appear haphazard, many of the species are quite common, so they must work well, with the number of eggs laid making up for losses through failure to find hosts.
How Do They Breathe?
Because the parasites are covered in the host’s body fluids, they have the same respiratory problems as aquatic insects. When very small, many can obtain sufficient air for their needs by direct diffusion from the host’s tissues, but as they grow bigger and their respiratory requirements increase, it is normal for them to seek out atmospheric air. This they do by boring either through the host’s skin or into a main tracheal trunk or spiracle.
Some species overcome this problem by remaining with their hind ends attached to the original entry hole. The host’s reaction to this trauma is to secrete a sheath around the parasitic larva. At first, the parasite feeds only on the host’s blood and fat, but when in their final larval stage, most of them rupture their sheaths and start feeding on the hosts’ vital organs, rapidly killing them.
Common Species Of Tachinid
The Tachinidae is an important family biologically and plays a role in controlling insect numbers only second to that of the ichneumons and their allied parasitic wasps. Several species have been used in the biological control of insect pests, with some being established in America to help control the gypsy moth.
One of the commonest and most ordinary-looking tachinid flies is Phryxe vulgaris, 6-9mm long and blackish with a whitish sheen on its abdomen when viewed from certain angles. This species and its relation, Phryxe nemea, have each been recorded as parasites of hosts belonging to 15 families.
Compsilura concinna is a very similar-looking fly but has been recorded from an even greater number of hosts, over 100 being known from 18 families of Lepidoptera.
Other common tachinid flies include Tachina grossa and T.fera, found respectively on umbellifers and water mint, and Siphona geniculata, a brownish-grey fly that is often seen probing for nectar on flowers from May to October.
Its larvae parasitize leather jackets (cranefly larvae). Gonia divisa is found in early spring when it can be seen flying low over the ground on heaths and commons. Its tiny eggs are eaten by the caterpillars of noctuid moths and occasionally by the larvae of bumble bees and potter flower bees. Both these hosts fly in early spring and are thought to carry the tiny eggs of Gonia back to their nest in pollen.