Insect guide > Flies > Bot Flies

Bot Flies

(Family Oestridae.)
This family contains the parasitic creatures known as bot­flies, several of which are the cause of great suffering and even the death of domestic animals. The bot-fly of the sheep (Oestrus ovis), the bot-fly of the horse (Gastrophilus equi), the bot-fly of the ox which is known in England as the"ox warble" fly (Hypoderma lineata, the European species being H. bovis), and certain other forms whose larvae live under the skin of such wild animals as squirrels and rabbits (genus Cuterebra) and which are sometimes in tropical regions found under the skin of human beings (Dermatobia cyaniventris) belong to this family.

The group is not a large one, comprising only about sixty species and the life history of the different species is quite variable, comprising many strange and curious phenomena.

All, however, are parasitic in vertebrate animals. The flies themselves are rather large, generally rather hairy, and they are as a rule inconspicuous in their coloration. The antennae are small and inserted in rounded pits.

Typical Life History
(Hypoderma lineata Villers.)
This fly is the common "ox bot" or "ox warble" of the United States and is known in the southwestern country as the "heel fly". To stock raisers its larva is also known as the "grub". Affected cattle are known as " grubby" cattle.

Early in the spring the flies appear and are immediately attracted to cattle, laying their eggs upon the legs, especially just above the hoof, which explains the southwestern name "heel fly". The eggs are occasionally laid on other parts of the body but the neighbourhood of the hoof is preferred. They are attached to the hair by means of a clasping projection and usually from four to six are laid together. The animal licks its legs and the larva at once hatches and is carried down into the oesophagus, the walls of which it penetrates by means of its strong spines. It then molts and becomes smooth and for several months wanders through the connective tissues of the cow, between the skin and the flesh, penetrating gradually along the neck and ultimately reaching a point beneath the skin on the back of the animal. The larva then molts again, becomes more spiny, and bores a hole through the skin, placing its anal spiracle near the orifice in order to get air.

During its earlier life it probably breathes by an endosmotic method as do the larvae of the parasitic Hymenoptera and in fact much as do the aquatic larvae of certain other insects. The larva now develops rapidly, living upon the pus and bloody serum which is produced by the irritation of its spiny skin. It molts again and is then more than an inch long and yellowish­white in color. It works its way out of the minute orifice which it enlarges and drops to the ground where it contracts and hardens, the larval skin becoming the protection for the pupa which is formed within. In three to six weeks the adult fly escapes by pushing off the circular cap at one end of the puparium.

The life history of this insect was entirely misunderstood. It was supposed that the eggs were laid upon the back and that the larva immediately penetrated the skin and lived there without wandering. It was not until 189o that the true life history, as described above, was ascertained by Dr. Cooper Curtice.