150 



KNOWLEDGE 



[July. 1902. 



tliioiiglunit the lusecta. Several othor insects, closely 

 iilliod to this one, are uiodifierl more or loss in the same 

 manner, each one, however, with a ]iattern peculiar to itself. 



The third type of fore-lep; is what. T have called the 

 raptorial, by which is meant that the peculiar formation 

 of the fore-legs has regard not to h)coTnotive requirements, 

 1)U< to the capture of living prey. In several instances the 

 l'(ire-legs are used for this ])urii<>se without any special 

 nioditication of form. This is notahly the case with the 

 groujt of water bugs called " sluitors," which may be seen 

 any day sitting on the surface of a pool, with their thin 

 long median and hind legs outstretched and ready to start 

 off at a moment's notice, skating over the water out of harm's 

 way. These two long hinder pairs of legs are set on at 

 right angles to the axis of the l>ody and in a horizontal 

 plane, in wliich the whole of their movements take place, 

 but the fore-legs, which are much shorter, ai-e set on in the 

 usual way, and as the insect sits upon the water, balancing 

 itself by its outstretched skating legs, its short fore-pair 

 rest with their tarsi only on the surface. And here we 

 see the advantage of having three pairs of legs. Two 

 pairs ai-e quite enough to steady the animal on the water 

 and keep it in position, and then the third pair is available 

 for any other duties, for in fact, these fore-feet seem to be 

 as useful as hands. By rubbing the antennae and beak 

 between them, these important organs are kept clean and 

 tidy, and further, these same limbs are useful for seizing 

 the flies and other small insects that frequent the surface 

 of the pond, and holding them in position while their 

 juicy bodies are being drained dry by the sharp beak of 

 their captor. No particular modification of form is 

 requisite in such a case. 



But the water scorpions, which live at the bottom of 

 the pond, and could not conveniently manage in the same 

 way, have their fore-legs very remarkably altered both in 

 form and in the direction in which they can move. As 

 one of these insects was described in our last paper, we 

 may now take as our illustration its much commoner rela- 

 tive, the Common Water Scorpion {Nejxi cinerea). This is 

 an abundant insect in almost any pond, at least in England. 

 It cannot possibly be mistaken ; the extreme flatness of its 

 body, the curiously curved shape of its fore-legs, and its 

 long tail filaments will enable it to be recognised at a 

 glance. The first peculiarity we notice about the fore-legs 

 (Fig. 3) is that they move horizontally, and hence would 



Fro. 3. — Head and Fore- Fio. k A. Fore-leg of Dung Beetle. 



leg of Water Scorpion. B. Ditto of Mole Cricket. c, eoxa ; 



A head; «, eyes; p, tborax; r, trochanter; s, femur; t, tibia; 



c, coxa; t, trochanter; /, /, tarsus, 

 femur; rf, tibia; a, tarsus. 



be useless for walking purposes ; this change in direction, 

 however, gives them a splendid power in thi> seizure of 

 prey, for, working from side to side as they do, their ends 

 meet in the middle and grip as by a pair of pincers any 

 object that happens to lie between them. The coxa;- are 

 stout and strong and project from the body straight 

 forward on each side of the beak. 1'he trochanter is much 

 larger than usual and servos to turn the corner so as to 

 enable the next part of the leg to stretch out sideways. 

 The lemur is by far the largest and longest i>art of the 



leg, and its inner edge carries a distinct groove, the use of 

 which will api>ear presently. The tibia and tarsus together 

 are only as long as the femur, and they are so pW:ed as 

 to curve round from the end of the femur towards each 

 other, so that they «;au meet straight in front of the head. 

 When not in use they are folded inwards along the edge 

 of tlie femur, fitting most accurately into the groove afore- 

 said, just in the same way as the blade of a pocket knife 

 shuts back into the handle. The tarsus itself ends in 

 a sharp-pointed claw. Such is the limb which by slight 

 alterations of form has been transformed from a simple 

 harmless walking implement into a murderous weapon by 

 which living creatures can be seized and brought to the 

 mouth, while their juices are abstracted by the short and 

 sharp beak. 



The next type of leg is the fossorial, or digging, one. 

 Many insects that burrow in the ground use their jaws for 

 nibbling away the earth, but others employ their legs, and 

 particularly the fore-pair, in scooping out the soil and so 

 working their way along, and there are certain modifica- 

 tions of structure by which a fossorial leg can always be 

 easily distingiiished. The many-jointed foot would of 

 course be too yielding to serve as the digging implement, 

 and hence the duty is handed over to the next part, viz., 

 the tibia, or shank. In digging insects the tibia is strong 

 and stout, and furnished on the outer edge with very hard 

 and powerful tooth-like projections. It is chiefly amongst 

 the order Coleoptera or beetles that these digging legs are 

 found, and they attain their most characteristic develop- 

 ment in tlie grou]i of dung-beetles (Fig. 4, a) — hard-bodied 

 insects that inhabit the droppings of horses and cattle, and 

 are most useful as scavengers. They habitually lie buried 

 in the dung, and their digging legs are of great assistance 

 to them in working their way into and through the mass. 

 In the case of the largest examples, such as the " Watch- 

 men," or " Dor " beetles, these legs further aid in an 

 elaborate process of well-sinking, which is needed for the 

 projier location of the eggs. Beneath the mass of dung, 

 the female beetle excavates a deep and very accurately 

 formed cylindrical burrow, at the bottom of which she 

 l^laces a store of the dung together with one of her eggs. 

 This is destined to hatch in about a week's time, and the 

 young grub therefore finds itself started in life with a 

 well-furnished larder, the contents of which it can proceed 

 lazily to enjoy. 



In all these digging beetles, the tarsi are a good deal 

 reduced in size and enfeebled in power, and one can readily 

 understand the possibility of their ultimat^'ly becoming 

 practically useless, and so being more or less aborted, for 

 as the tibiiB increase in size and power, the insect would 

 depend more and more upon them and less and less upon 

 the tarsi, which, indeed, might, it would seem, get in the 

 way of the digging operations. Such a stage as this has, 

 in fact, been reached in another insect, belonging to quite 

 a diii'erent order, the mole-cricket {Gryllotaipa viil(jaris),a. 

 member of the Orthoptera. This insect is as well adapted 

 for burrowing as the little quadruped from which it gets 

 its name, and the modification of the fore-limbs has, in the 

 two cases, proceeded on similar lines. In the mole-cricket 

 (Fig. 4, b), the tibia is enormously broad and strong, and 

 ends in four very powerful teeth which give it the 

 appearance of a hand, and it forms as good an excavating 

 implement as the wonderful hand of the mole itself. The 

 muscles required to work this powerful implement need 

 safe lodging and strong attachment, and hence the femur 

 too is widely dilated, and is almost as stout and strong as 

 the tibia. On the other hand, the tarsus has dwindled 

 away to a most insignificant size, and has been shunted on 

 one side where it lies underneath the tibia ; in fact, it is 

 not likely to be noticed at all except on close inspection, 



