November, 1902.] 



KNOWLEDGE 



259 



Then comes the I'mg and thiu shank, which is succeeded 

 by a foot, the like of which we may search for elsewhere 

 in vain. Instead of the pair of claws by which an insect's 

 foot is usually terminated, there is an apparatus propor- 

 tionately very large and most awkward looking, and 

 reminding one of a boy's clasp knife (Pig. 3). Being in 

 reality a modified pair of claws, it consists of two parts, 

 one of which is broader, turned up at the tip, and fur- 

 nished with stiff bristles along its inner edge, and this 

 corresponds to the hasp of the knife ; the other is thii). 

 blade-like and pointed, and represents the blade. The 

 claw-like blade can be opened out fully so as to be in 

 a line with the stouter hasp, or it can be completely closed 

 upon the latter just as the blade upon the hasp. The 

 whole apparatus is attached to the rest of the foot at the 

 hinge where its two parts move upon one another, and, 

 therefore, though symmetrically disposed with regard to 

 the foot when open, it hangs down on one side when 

 closed, and gives a most unsymmetrical and malformed 

 appearance to the limb. This curious apparatus acts as a 

 pair of pincers, and its function is evidently to seize hold 

 of the bristle-like hairs of some froghopper less wary 

 than usual, so that it may be detained to serve as a host 

 for the egg of the parasite. It can hardly seize anything 

 much larger than the hairs of its cjuarry, as the whole 

 apparatus is so minute that a microscope is necessary in 

 order to make out its details. 



For our ne.xt oddities we will go to the order Dipt«ra, 

 or two- winged flies. There is a certain section of this 

 order the members of which are external parasites on 

 warm-blooded vertebrates, and as habits of this kind often 

 produce great modification in the form of the parasite, we 

 need not be surprised that some of the insects referred to 

 are so changed as to be scarcely recognisable as flies. 

 They are generally Ihit-bodied, shiuy creatures, with 

 bristles scattered over the 

 bodv, and with stout legs 

 ending in strongly curved 

 and very powerful claws. 

 The single pair of wings 

 which distinguish dip- 

 terous insects are usually 

 present, though often 

 greatly modified in form ; 

 but sometimes they are 

 altogether absent, and the insect is quite apterous, so 

 that none but an expert would suspect its near relationship 

 to the active and vivacious flies. 



Our first example may be the " bird-fly," Oniithomyia 

 aviciilaria, which is a parasite on birds, and sucks their 

 blood ill much the same way as the bed-bug does that of 

 humankind. This insect sIkjws very well the Hattened 

 head and thorax which are so common in the group ; its 

 wings are ample and it can fly well. The claws are large 

 in proportion to the size of the body, and they are very 

 powerful and permanently bent abruptly inwards, re- 

 minding one of the corresponding structure amongst 

 vertebrates in the hand or foot of a sloth. The claw of 

 an allied insect is shown at Fig. 5a. This arrangement, 

 while it unfits the insects for walking easily on flat 

 surfaces, gives them a wonderful tenacity of grip on un- 

 even surfaces, and makes it difficult to dislodge them from 

 the feathers of the birds on which they live. They are 

 very vivacious insects, running with great agility, often 

 sideways like a crab, and easily eluding pur.snit by burying 

 themselves in the feathers of their host. These parasitic 

 flies, though exhibiting the habits of lice, must not bo 

 confoimded with the true bird-lice, which are very different 

 and much smaller insects, apterous and constituting a 

 distinct order called Mallophaga. 



FlO. 3. — Claws of Go la/opiu 

 pedettris. Highly magnified. 



A rather larger insect, somewhat similar to the bird-fly, 

 but with dark shiny brown body, attacks horses in the 

 same way, and is sometimes exceedingly troublesome to 

 them. In accordance with its habits, it is called 

 Hippobosca equiin. It has a very tough body which can 

 staud a great amount of pressur ; without being harmed — 

 a very useful attribute when one thinks of the risks it 

 must often run in getting into narrow corners, under 

 harness, &c. The head of this insect is worth a careful 

 examination under the microscope, as it shows the sort of 

 structure an insect often takes on in becoming parasitic. 

 Everything is arranged with a view to the least resistance 

 in passing amongst the hairs of the animals on whose 

 blood it feeds. The surface of the skin is very polished 

 and well rounded, without prominences which might get 

 in the way. The outline of the head is continued in the 

 eyes, and the antennae are neatly disposed of by being 

 sunk in a couple of pits. They usually stand out promi- 

 nently on a fly's head, and if that were so in- the present 

 case, they would no doubt be an impediment to rapid 

 progress amongst a forest of hairs. But they are much 

 shortened and consist mainly of a large round knob, 

 surmounted by a long and strong black bristle, which is 

 the only part visible outside, all the rest being sunk in the 

 pit. It is curious that an exactly similar contrivance 

 may be found in another parasite, though a very dis- 

 similar one, viz., the common flea. Between the antenna;, 

 and at a little lower level, is a sort of black beak which 

 ensheaths the piercing apparatus. This latter contains 

 bristles strong enough to penetrate even the tough hide of 

 a horse, and thereby cause intense irritation. 



The swallow louse, Stenoptenj.v hirunduiis (Pig. 4), is 

 another of these parasitic flies. It has the same flattened 

 and polished body, and the same 

 stout legs and curved claws, the 

 former green and the latter black, 

 as usual. But it has advanced a 

 stage on the road to the apterous 

 condition ; the wings are reduced 

 very much in breadth, and look as 

 if all the inner parts had been cut 

 away with a pair of scissors, only 

 the outer and stronger edge being 

 left. The creature lives parasitically 

 upon swallows, making its way, crab 

 fashion, amongst the feathers till it 

 reaches the skin, which it pierces with 

 its beak for the sake of the blood. If 

 the host dies, the fly is not long in 

 discovering the fact, and it forsakes the now useless body, 

 flying away till it encounters some other living being on 

 which to alight and try the strength of its beak. If this 

 does not happen to be a bird, the fly soon leaves it and 

 proceeds on its travels tUl a more suitable host is dis- 

 covered. When thus free, it is sometimes troublesome to 

 human beings. The Rev. W. Kirby mentions a case in 

 which one of these creatures behaved just like a bed-bug, 

 establishing itself in a bed and greatly worrying the 

 occupant for several nights without his su.specting the 

 true nature of the annoyance, till at last a close and 

 careful searcli brought the culprit to light. Curiously 

 enough, when I had reached this point in writing 

 this paper, a living specimen of this same fly was 

 brought me, which had settled upon a person walking 

 by the side of a wood, and had startled him by its 

 quaint form and strange motions, so that he captured it 

 at once. 



This swallow louse, as mentioned above, has got its 

 wings abbreviated, and it seems likely that in the course 

 of time its descendants may have theirs still further 



Fie. 4. - Swallow 

 louse (Stenopteryx 

 hirundini^). 



