28 



KNOWLEDGE. 



[December 2, 1889. 



dung, &c. That tlieir affinities are strongest with the 

 Diptera is now generally recognised, and they are therefore 

 regarded as a sort of apterous Hies, which have addicted 

 themselves to parasitic habits. The reasons for this 

 opinion will become evident as we proceed. 



Having premised thus much as to the zoological posi- 

 tion of the group, we may now endeavour to get a clear 

 notion of the structure of I'lih:!- initniis {Vi'^. 1 i. the 



Ki(i. 1. — Common Flea (Pules in itans), female ; e, epimeron : ■ 

 .>■, tibia ; p, tarsus. 



common human ilea, so-called, and afterwards deal with 

 its life history. In the shape of their body, fleas are quite 

 exceptional ; it is flattened from side to side, so that when 

 the insect is standing upright its greatest diameter is the 

 vertical one ; this form of body is called " compressed " ; 

 it is exactly the reverse of what obtains in that other 

 nocturnal pest, the bed-bug, whose body is flattened in the 

 vertical direction, and whoso greatest diameter is the 

 transverse; this shape of body is called "depressed." 

 Certain fishes present similar extremes of structure ; thus 

 a skate is depressed, but a plaice or sole compressed. The 

 flea's body is covered with a hard, slippery, reddish brown 

 ehitinous skin, showing plainly enough the division into 

 rings or segments which characterises insects and other 

 annulose aniuials. The head, wliich is rounded above, is 

 small in proportion to the size of the insect, and is fol- 

 lowed by three small and separate segments which repre- 

 sent the thorax ; these are again succeeded by several 

 much larger segments forming the abdomen, which, in the 

 female, is at least three times as deep as the head. To 

 the thoracic segments are attached, as usual, the three 

 pairs of legs, which increase m length from before back- 

 wards. 



The legs are remarkable in several ways. At first sight 

 they seem to have an extraordinary number of joints, and 

 yet the parts are exactly the same as in insects generally, 

 and follow the plan typified in the cockroach. It will be 

 remembered that that joint of an insect's leg by which the 

 limb is attached to the thorax is called the coxa. Now 



the coxiB of a flea are not only enormously large, being 

 indeed the broadest and almost the longest section of the 

 leg, but they are also far more completely freed from the 

 thorax than is usually the case, being only attached by one 

 extremity ; this causes the leg to appear to have an extra 

 joint. But this is not all ; in the first pair, especially, we 

 seem to have yet another additional joint, and this appear- 

 ance is due to the fact that the ijiinn-ra (\iz. those elements 

 of the thoracic segment 

 to which the coxse are 

 directly attached) them- 

 selves project from the body 

 of the segment, and point 

 oliliilHcly forwards. These 

 arrangements give an ex- 

 tremely awkward appear- 

 ance to the legs, but no 

 doubt facilitate the leaping 

 process. The trochanter is 

 small, and both fenuir and 

 tibia are about the same 

 length as the coxa ; but 

 the tarsus, which, like that 

 of the cock-roach, consists 

 of five joints, is remarkably 

 long, and is terminated by 

 a pair of long curved claws, 

 which the insect must find 

 extremely useful as it works 

 its way about amongst the 

 garments of its host, or 

 between the bed-clothes. 

 Most leaping insects have 

 the hind femora very 

 largely developed, since in 

 them are placed the mus- 

 cles which originate the 

 impulse of projection : this 

 arrangement is especially 

 noticeable in grasshoppers, 

 and in the tiny beetles called turnip fleas, winch do so 

 much harm to cruciferous plants. The hind legs of 

 the flea, however, scarcely difi'er from the other pairs 

 except in length, and the proportionate dimensions of 

 all three pairs are much the same, all having coxse 

 larger than would be requisite for a walking insect. 

 All parts of the legs are beset with bristly hairs, those 

 towards the end of the tarsus being especially closely 

 packed. The abdommal segments also are furnished 

 with bands of long, still' hairs across the back. No 

 doubt these hairs — all pointing, as they do, away 

 from the head — aid the flea, quite as much as its com- 

 pressed form, in its endeavours to insinuate itself into 

 the small spaces between our garments it has oiten to 

 travel along in order to reach its pastures ; and help, at 

 the same time, to explain the difficulty that one experience s 

 in attempting to hold the insect between finger and thumb. 

 Turning now to the mouth organs (Fig. 2), we find a 

 far more complicated apparatus than might have been 

 expected. The type of mouth is that called suctorial ; /.<■. 

 it is adapted, as we are painfully aware, for the swallowing 

 of liquid food, obtained by a process of perforation. In 

 this respect fleas agree with flies, and, for the matter of 

 that, with bugs ; but are totally unlike bees, wasps, and 

 ants, to which group of insects some people have thought 

 they show some affinity. The labrum, or upper lip, seems 

 to be represented only by a slender saw-edged bristle, 

 which is perforated throughout its length by an exceed- 

 ingly minute canal. This is situated in the centre, and 



/, trochanter ; /', feuuir ; 



