Fig. 1. The larva of a flea. The body consists of thirteen segments and is legless. On the fore part of the head are the antennæ and on the upper part of the head is shown the knife-like edge of the egg-breaker. The mouth-parts are adapted for biting. On the last segment of the body are the two caudal stylets.
The larval stage lasts some days, and the animal spins a small cocoon before pupating. In the course of a few more days, the time probably depending on the weather, the perfect flea emerges. The larvæ generally live in places where the perfect insects will have an opportunity of finding a host as soon as they leave the pupal envelope. The nests of their hosts where the young are being reared are always favourite places. It seems possible that the comparative immunity from fleas which hoofed mammals or Ungulates enjoy may be due to the fact that the young beast follows its mother from the time of birth instead of passing its early life helpless in a nest.
Observations made on the development of the dog-flea (Ctenocephalus canis) in India show that eggs laid on October 17 hatched on October 19. The larva spun its cocoon on October 25 and the mature flea emerged on November 2. In Northern Europe the human flea takes about four weeks in summer and six weeks in winter to pass through its metamorphosis.
Unlike many parasitic insects, fleas do not constantly pass their time upon the bodies of their victims. The greater part of their life is probably spent on the ground, in the house, or nest, of the mammal or bird which serves them with blood. In this respect there is considerable difference in the habits of different species of flea. Some attach themselves to an animal and actually burrow into the skin. These are the most parasitic species. Some only come to feed and leave to lay their eggs. Many probably do not suck blood more than once in their lives.
An animal which harbours fleas and which nourishes the adult insect with blood is called a host. No fleas are more than what is called temporary parasites; which means that they pass but a portion of their lives on their hosts and frequently take occasion to hop on and off. All fleas, apparently, go from host to host. The labours of diligent collectors have proved that the great majority of mammals and birds have fleas. As a general rule, it is true to say that certain species of flea are associated with certain species of host. Thus man is the true host of Pulex irritans; the cat family are the true hosts of the cat-flea (Ctenocephalus felis); and the dog family are the true hosts of the dog-flea (Ctenocephalus canis). But the human flea is sometimes found on cats and dogs, and cat and dog-fleas occasionally bite human beings; and cat-fleas are found on dogs and dog-fleas are found on cats. All fleas, so far as we know, may occasionally pass from one species of host to another; but they do not, for the most part, seem to flourish in unaccustomed quarters. Some fleas are more catholic in their tastes than others. Some seem to be very strictly confined to one host, and even when starving only suck strange blood under protest. There is a species of flea that has only (except by accident) been found on the long-tailed field-mouse and another that has only been found on the hedgehog. Other fleas are commonly found on two absolutely distinct animals; a good instance of this is the human flea which, at all events in certain parts of England, is a regular parasite of the badger.