INSECTA. 



or the knee-joint, as we might term it. Upon the upper ex- 

 tremity of the tibia the observer will find on each side a precise 

 semicircular furrow, behind which is a concentrical but smaller 

 ridge, and still further back a circular depression or fossulet. On 

 examining the corresponding surfaces of the femur, he will detect 

 a ridge accurately corresponding to the above-mentioned furrow ; 

 behind this a furrow corresponding to the preceding ridge, and still 

 further back, a minute elevation adapted to the fossulet of the 

 tibia, wherein it is fastened by a minute but very strong ligament. 

 Such ridges and grooves, when fitted into each other, form a joint 

 evidently admitting of a free and hinge-like motion, while from 

 its structure, dislocation is almost impossible. 



(283.) The above general description of the leg of an insect will 

 prepare us to examine various modifications in outward form and 

 mechanical arrangements by which these simple organs are adapted 

 to progression under a great diversity of circumstances. When, 

 indeed, we reflect how extensively this class of animals is distri- 

 buted, and the variety of situations in which insects live, we are 

 led to expect corresponding adaptations in the construction of their 

 instruments of locomotion ; and in this our expectations will not 

 be disappointed. 



In the generality of terrestrial species, the last segment of the 

 tarsus or foot is provided with a pair of strong horny hooks, which 

 are available for many purposes, being used either for creeping 

 upon a moderately rough surface, for climbing or for clinging to 

 various substances. 



Such simple hooks, however, would not always serve. In the 

 case of the louse (Pediculus) for example, that is destined to 

 climb slender and polished hairs, such prehensile organs could be 

 of little use. The structure of the foot is therefore modified ; the 

 tarsus in this insect terminates in a single moveable claw, which 

 bends back upon a tooth-like process derived from the tibia, and 

 thus forms a pair of forceps fitted to grasp the stem of the hair and 

 secure a firm hold. 



Many insects, especially those of the Dipterous order, are able 

 to ascend the smoothest perpendicular planes, or even to run with 

 facility, suspended by their feet in an inverted position, along sub- 

 stances which, from their polished surfaces, could afford no hold to 

 any apparatus of forceps or booklets. In the common flies (Mus- 

 cidcc), the exercise of this faculty is of such everyday occurrence, 

 that, wonderful as it is, it scarcely attracts the attention of ordinary 



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