228 THE AMEKICAN MONTHLY [December, 



■whole flea tribe. Then we have one species, P. Irritans^ which attaches itself 

 specially to man himself, though of course it only selects the dirtier classes 

 of men. I should, perhaps, add the Jigger or Chigoe to my list ; it also is 

 a species of flea. Packard speaks of it as "one of the most serious insect 

 torments of the tropics of America." 



You will see from all this that any one who wishes to get together as many 

 different fleas as he can may secui"e at least two dozen slides while he is 

 about it. I would suggest that he should make drawings of the five tarsal 

 joints of his various specimens, for here he will find some unexpected dif- 

 ferences of minute structure. For example, taking them in the order of 

 greatest length, the tarsal joints in the anterior legs of both the cat and dog- 

 flea would be registered 5, 2, i, 3, 4; but in the cat-flea the posterior joints 

 would be I. 2, 5, 3, 4, while in the dog-flea they would be i, 5, 2, 3, 4. 

 Moreover, the dog-flea is adorned with a distinctive collar, a pectinate fringe 

 on the pro-thorax, the badge, we may suppose, of his order ; and in addition 

 to this he wears a row of formidable brown setcB on the lower part of his 

 head, both of which characteristics are wanting in the cat-flea. Now, these 

 are not quite such small matters as they seem. For one thing, they show 

 how materially the forms of two fleas, which infest two different but closely 

 associated domestic animals, are subtly affected by their surroundings and 

 habits and the general conditions of their lives. Here, then, is matter for 

 instructive reflection furnished by the different fleas in a microscopist's 

 cabinet. Believe me gentlemen, if you merely treasure up your objects 

 for exhibition to friends and don't yourselves seriousl}- study them, your 

 specimens will not be a whit more useful to you from a scientific point of 

 view — even though you actually need a good microscope for their exhibi- 

 tion — than would be a collection of postage stamps or of military regi- 

 mental buttons. Indeed, in these hard times, stamps and buttons would be 

 preferable, because the mania for collecting them would save you at least the 

 cost of a microscope. In the event, too, of a re-sale, I venture to affirm 

 that a good collection of postage stamps would be sure to realize a higher 

 price than — two dozen old fleas. 



To proceed, the notes on which this paper is based are taken from my 

 diary for 1SS6, and refer to observations then made on some fleas' eggs which 

 were deposited early on the morning of the 17th October, 1S86. You will 

 find a sketch of one of the empty ^^% cases amongst the drawings in fig. 2 of the 

 book I now hand round. If you spread a cloth, 2^ jharun is what you want, 

 and let a dog sleep on it, you will find with the help of a pocket lens that the 

 dust, etc., which he leaves behind him includes fragments of cuticle, hairs, 

 fibres, and above all, nits and minute pellets of dried blood, which are probably 

 the natural excreta of the flea. I dusted such a cloth on to a sheet of paper, 

 and roughly separating the eggs and blood pellets from the other materials 

 present, put the former into an ordinary finger-glass, which I covered with a 

 broken pane of glass. On the morning of the 19th October, about 50 hours 

 after deposition, most of the nits had hatched out, though a few took a day 

 or too longer. The period of incubation, therefore, for the larger number of 

 eggs collected by me, was a trifle over two days. The larvae were white, 

 cylindrical, active grubs. Their bodies seemed to me to be in 13 segments, 

 exclusive of the head. The larva is eyeless : and according to Packard, the 

 shape of its head, its habits of living in dirt, its movements, and its transfor- 

 mations are held to ally the flea to the Mycetophilids, a group of fungivorous, 

 two-winged flies, which are endowed, like the flea, with considerable leaping 

 powei". Kindly bear in mind in passing what I said about the flea which is 

 found in brush-wood, and that other flea which affects a species of mushroom. 

 Fleas, though formerlv placed in an order by themselves. Aptera, are now 



