JoLY, 1902.] 



KNOWLEDGE 



149 



femur bends up from the coxa and trochanter, the tibia 

 liends down from the femur, and the tarsus bends up 

 again from the tibia, so that the angles caused by the 

 bendiuLT are alternal«lv' on opposite sides of the leg. The 

 figure shows this verv well. 



A little thought will show that the exact method in 

 whiih these legs are used must differ according to the 

 yiosition of the leg on the body. Koughlj we may say 

 that the fore-leg pulls (he body forward, while the hind- 

 leg pushes it, and the middle one serves as a sort of jiivot 

 or biUanciug jioiut. The general rule in walking or 

 running seems to be that the first and third legs of one 

 side, and the middle one on the other, are put forward 

 simultaneously, or nearly so, for the first step, and then 

 are followed by the other three for the next, so that the 

 insect proceeds by means of two tripods which are planted 

 on the ground alternately. In taking a step forward 

 therefore, the right fore-leg, say, will be advanced, and 

 with it or immediately after it the left midille leg and the 

 right hind leg. In advancing the fore-leg, however, the 

 limb is of course stretched i>ut to the full, and as the body 

 moves forward, the joints of the legs close up. The 

 reverse is the case with the hiud leg. This is in the 

 contracted condition at the beginning of the step, and is 

 opened out as the step proceeds, thus of course acting as 

 a thrust behind, while the fore-leg e.verts a pull in front. 



As a rule these movements are effected with very great 

 rapidity, so that the eye can hardly follow them, and 

 hence it is no easy matter to verify the above statements 

 by merely watching a moving insect. A good deal of 

 j>atieuce and mental concentration is required to watch 

 six moving objei:ts simultaneousl}', .and determine the 

 order of their movement, even when it is not very rapid, 

 and in most insects it is far too rapid to admit of a 

 determination in this way. If, for example, the observer 

 were to try the experiment on a kitchen cockroach, following 

 it about and keeping his eyes steadily upon it all the time, 

 he would probably be conscious merely, of a dazing motion, 

 tiring to the eye and defying analysis, and his patience 

 would be exhausted long before he could satisfy himself 

 as to the true scheme of progression. But by photo- 

 graphing the insect at exceedingly short intervals of time, 

 a minute fraction of a second in fact, on a revolving film, 

 a series of silhouettes is obtained of the different phases 

 of a single step ; this is evidently equivalent to a slowing 

 down of the movement till it can be followed with the eye, 

 and thus it is possible to determine the order in which the 

 legs are advanced and progression takes place. 



Our second type of leg is that which may be described 

 as the prehensile one. This is a sexual characteristic 

 confined to the males, and is found fully developed only in 

 a small number of insects. The foot is converted into a 

 sort of clasping organ, and, in fact, becomes a kind of 

 hand, by which the suitor is able to secure and retain his 

 mate. In cerhvin water beetles the mt^thod of adaptation 

 is that of a great expansion of the basal joints of the 

 tarsus, whereby a kind of suction pad is formed which is 

 sometimes of great Ijeauty and delicacy of construction. 

 The most remarkable and beautiful example is that of the 

 great carnivorous water beetle (Dytisciis manjinalis). In 

 the male of this insect (Fig. 2, a) the three basal joints of 

 the tarsi are enormously broadened and made to fit close 

 together so as to form a circular pad. On the upper 

 surface there is nothing very remarkable about this exce])t 

 its size ; it looks as though the insect had been a martyr 

 to rheumatism or gout, and so had got its joints swollen 

 beyond recognition. • But on the underside these pads are 

 marvels of complexity and grace. Their surface is very 

 thickly set with what appear to be extremely modified 

 hairs. Two of these form circular cup-shaped discs, the 



largest of which is about Jj inch in diameter. The 

 circumference of the discs is margined with fringed hairs. 

 The rest of the surface is closely covered with very much 

 smaller trumpet-shaped hairs, which are present in con- 

 siderable numbers, and the whole pad is thickly fringed 

 with simple hairs round its edges. All this ci>llection of 

 trumpets, cups, and fringes acts as a complex sucker, and 



I If 



Fig. 2 — A. Fore-foot of male Dytiscus. (i) Uppor-siJe. 

 (ii) Under-side. 

 B. Forejeg of male Crabro crihrarius. (i) Tibia, 

 (ii) Tarsus, t, tibia; .«, shield attaelied to tibia; 

 f, thigh ; p, projection from tliigh boneatli the 

 shield, appearing faintly through it ; c, claw. 



adheres to the back of the female insect, the surface of 

 whose wing-covers is roughened by longitudinal furrows, 

 and thus rendered easier to hold. Another very common 

 water-beetle, flatter and rounder in shape, and called 

 Acilins sulcatus, is furnished with similar fore-feet, but 

 these two, with perhaps another common Dyliscus, are 

 almost the only insects commonly met with that possess 

 this peculiar apparatus. 



In the great black vegetarian water beetle, Hydropkilus 

 2)ictiis, a slightly different arrangement occurs ; its fore- 

 feet are modified, it is true, and for similar purposes, but 

 in a different wa}-. Here it is the last tarsal joint that is 

 enlarged, and not the basal ones. This last joint forms a 

 great triangular disc, but it is not furnished with trumpets 

 and suckers. 



Amongst the order Hymenoptera, a very peculiar 

 modification may be seen in certain species of the so-called 

 " sand-wasps." Ctabro cribrarius is one of the most 

 remarkable of these. It is a shining jet-black insect with 

 its body banded with bright yellow ; it is commonly found 

 in sandy localities, where it forms its burrows in the 

 ground, and in the cells it constructs for the rearing of its 

 brood, it stores uji flies of different kinds as food for the 

 young larvaj which will in due course be hatched from its 

 eggs. The male of this insect has so extraordinary a pair 

 of fore-legs (Fig. 2, b) that when one first sees them one 

 finds it difiicult to believe that the insect is not suffering 

 from a deformity, and it is only on noticing the distinct 

 outlining of the different [)art8 and the exact correspondence 

 of the two legs, that one becomes convinced that one is 

 dealing with a natural structure. The whole length of the 

 leg is affected more or less, and every joint is modified in 

 some extraordinary way. The thighs are short and 

 swollen, and produced behind into a most fantastic, five- 

 sided and rather twisted projection, near to which is a 

 sharp spine. Then the tibiae are expanded all along their 

 outer edge, so as to make a large shallow cup like a shield, 

 the surface of which is discovered, on holding it u\) to the 

 light, to be a kind of network with the meshes filled in for 

 the most part with a dark, but here and there with a 

 transparent membrane ; the aforesaid projection from the 

 thighs runs into the hollow of this shield, and looks as 

 thoiigh it were intended as a support for the latter; the 

 tarsi again are much dilated, and they terminate in two 

 unequal claws, the inner one of which is very long and 

 curiously twisted. The whole apparatus is about as 

 remarkable and complicated as anything to be found 



