i4i 



CONCEENING FLEAS. 



The flea shares with the cockroach the doubtful honour of being almost as 

 common as the dirt in which both live ; but whereas the cockroach is useful as 

 a scavenger, the flea is harboured by the dirt at certain periods of its life- 

 history, which was carefully worked out, nearly - two centuries ago, by an 

 Italian, D'Jacenti Centone, whose observations are contained in a letter to 

 Martin Lister, published in the Philosophical Transactions for 1699, accompanied 

 by a plate of well-drawn figures of the flea, its eggs, the larva and cocoons. 



No one requires to be told of the general external anatomy of the flea. We 

 all know its abominable bilateral flatness, which makes it impossible to kill it 

 by pressure between the fingers. Every one, too, is acquainted with its wonder- 

 ful agility and strength, as expressed in the great leaps it takes when chased. 

 The leap has been measured, and it is found to be some two hundred times the 

 length of the insect, which is equivalent to a man leaping three or four hun- 

 dred times his own height. Ere this fact was scientifically known, Aristophanes 

 had held Socrates up to ridicule in attributing to him the endeavour to measure 

 this leap of the flea. 



The great sensitiveness to danger, the apprehension of being chased, is due 

 in great part to the sensory hairs which are set in rows across the flea's back ; 

 some of these hairs are much stronger than others, and these rows of bristles, 

 having different positions in different species, are of considerable value for 

 identification. 



If the head of a flea be examined from the side, the mouth organs depending 

 from its under-surface are seen to consist of two pairs of jointed sensory 

 appendages (the maxillary palps and the labial palps), by means of which the 

 animal can select dainty spots wherein to thrust its stylet-like mandibles. 

 These latter organs lie side by side in an incomplete tube formed by the 

 approximation of the two labial palps ; between the mandibles lies a median 

 piercer — the hypo-pharynx — ^which is traversed by the duct of the salivary 

 glands ; these probably discharged some irritant into the puncture. 



The flea lays some eight or ten eggs at a time ; these, in the case of the dog 

 or cat, are deposited among the fur ; in man in the undergarments ; but in 

 most cases the eggs, sooner or later, fall to the ground, and here undergo 

 development. Each egg is oval, and measures about 0*6 mm. in length. From 

 it there soon issues a white footless grub or larva, which breaks through the 

 egg shell by means of a " shell piercer " on its head analogous to that on the 

 tip of the beak of a young chick. This worm-like larva consists of a yeUow 

 head followed by twelve white segments, each provided with long sensory 

 hairs at the sides ; the large terminal segment carries a couple of stiff spines 

 by means of which, no doubt, locomotion is in part effected. The head carries 

 a couple of strong mandibles, by means of which the larva, which wriggles 

 about actively, feeds on decaying vegetable and animal refuse. This stage 



