There are one or two structural features in a flea which are peculiar: the most remarkable being that, unlike most other insects, it is much taller than it is broad. As a rule, insects—such as a cockroach, the bed-bug, or a stag-beetle—are like skates, broader than they are thick, but the flea has a laterally compressed shape, like a mackerel or a herring. Then, again, the three segments or rings which come after the head are not fused into a solid cuirass or thorax as they are in the fly or the bee, but they are movable one on the other. Finally, it is usual in insects for the first joint of the leg to be pressed up against and fused with those segments of the body that bear them; but in the flea not only is this joint quite free, but the body-segment gives off a projection which stretches out to bear the leg. Thus the legs seem, unless carefully studied, to have an extra joint and to be—as indeed it is—of unusual length. They certainly possess unusual powers of jumping—as Gascoigne, a sixteenth-century poet (1540–78) writes, ‘The hungry fleas which frisk so fresh.’
The male, as is so often the case amongst the Invertebrata, is much smaller than the female. The latter lays at a time from one to five minute, sticky, white eggs, one-fortieth of an inch long by one-sixtieth broad. They are not laid on the host, but in crevices between boards, on the floor, between cracks in the wainscoting, or at the bottom of a dog-kennel or in birds’ nests. Mr. Butler recalls the case of a gentleman who collected on four successive mornings sixty-two, seventy-eight, sixty-seven, and seventy-seven cat-fleas’ eggs from the cloth his cat had slept upon. Altogether 284 eggs in four nights! The date of hatching varies very much with the temperature. Pulex irritans takes half as long again—six weeks instead of four—to become an adult imago in winter than it does in summer. But in India the dog-flea will complete its cycle in a fortnight.
Fig. 8.—Pupa of flea. (After Westwood.)
When it does emerge from the egg the larva is seen to be a whitish segmented little grub without any limbs, but with plenty of bristles which help it to move about; this it does very actively. There are two small antennae and a pair of powerful jaws, for the larva does not take liquid food, but eats any scraps of solid organic matter which it comes across: dead flies and gnats are readily devoured. The larva casts its skin several times, though exactly how often it moults seems still uncertain.
After about twelve days of larval existence it spins itself a little cocoon in some sheltered crevice, and turns into a whitish inert chrysalis or pupa. During its pupal existence it takes, of course, no food, but it grows gradually darker, and after undergoing a tremendous internal change, breaking down its old tissues and building up new ones, the chrysalis-case cracks and the adult flea jumps out into the world.
There are many superstitions about fleas. March 1st is in some way connected with them, and in the south of England the house-doors are in some villages closed on that day under the belief that this will render the building immune for the following twelve months. The most successful insecticide is said to be prepared from Pyrethrum, which is grown in the Near East in large quantities for this purpose. But the Austrians, the Serbians, and the Montenegrins are fighting over the chief world-supply of this plant—possibly without knowing what they are doing—and ‘Insektenpulver’ is bound to go up in price. Wormwood (Artemisia) is also recommended.
While wormwood hath seed, get a handfull or twaine,
To save against March, to make flea to refraine;
When chambere is swept and wormwood is strowne,