942 



INSECTA. 



recti (c, c) on the upper surface, and the corres- 

 ponding ones on the ventral, which are now 

 chiefly subservient to the motions of the organs 

 of generation. 



From the number and complexity of the 

 muscles in these " miniatures of creation" 

 insects we feel less surprised at the agility of 

 their movements and the variety of motions 

 which many of them perform, and less asto- 

 nishment at the wonderful strength which 

 many species possess. But still there are 

 instances of some of them possessing a degree 

 of power that is almost incredible. The great 

 stag-beetle, Lucanus cervus, which tears off 

 the bark from the roots and branches of trees, 

 has even been known to gnaw a hole an inch in 

 diameter through the side of an iron canister in 

 which it was confined, and on which the traces 

 of its mandible were distinctly visible, as proved 

 by Mr. Stephens, who exhibited the canister at 

 one of the meetings of the Entomological So- 

 ciety,* an indication of an amount of strength 

 possessed by these insects of which before we 

 could have had no conception. But hardly 

 less surprising is the strength possessed by 

 Geotrupes stercorarius, which can support un- 

 injured, and even elevate an immense weight, 

 and make its way beneath almost any amount 

 of pressure. In order to ascertain the amount 

 of strength possessed by this insect, we have 

 made a few experiments from which it appears 

 that it is able to sustain and escape from 

 beneath a pressure of from twenty to thirty 

 ounces, a prodigious weight when it is remem- 

 bered that the insect itself does not weigh even 

 so many grains. But this amount of strength 

 is not confined to the short thick-limbed beetles. 



We once fastened a small Curabus ( ?), 



weighing only three grains and a half, by means 

 of a silken thread to a small piece of paper, 

 upon which the weight to be moved was placed. 

 At a distance of ten inches from its load the 

 insect was able to drag after it, up an inclined 

 plane of twenty-five degrees, very nearly eighty- 

 five grains. But when placed on a plane of 

 five degrees it drew after it one hundred and 

 twenty-five grains exclusive of the friction to 

 be overcome in moving its load. 



The motions of the insect in walking as in 

 flying are dependent in the perfect individual 

 entirely upon the thoracic segments, but in the 

 larva chiefly upon the abdominal. Although 

 the number of legs in the former is always six, 

 and in the latter sometimes so many as twenty- 

 two, progression is simple and easy. Miiller 

 statesf that on watching insects that move 

 slowly he has distinctly perceived that three legs 

 are always moved at one time, being advanced 

 and put to the ground while the other three 

 propel the body forwards. In perfect insects 

 those moved simultaneously are the fore and 

 hind feet on one side and the intermediate foot 

 on the opposite, and afterwards the fore and 

 hind feet on that side and the middle one on 

 the other, so that, he remarks, in two steps the 



* Transact. Ent. Soc. Lond., vol. ii. Journal of 

 Proceedings, p. xxii. 

 t Elements of Physiology, p. 970. (Transl.) 



whole of the legs are in motion. A similar 

 uniformity of motion takes place in the larva, 

 although the whole anterior part of the body is 

 elevated and carried forwards at regular dis- 

 tances, the steps of the insect being almost 

 entirely performed by the false or abdominal 

 legs. 



Inflight the motions depend upon the meso- 

 and meta-thoracic segments conjointly, or en- 

 tirely upon the former. The sternal, episternal, 

 and epimeral pieces, freely articulated together, 

 correspond in function with the sternum, the 

 ribs, and the clavicles of birds.* The thorax 

 is expanded and contracted at each motion of 

 the wings, as in birds and other animals, and 

 becomes fixed at each increased effort as a 

 fulcrum or point of resistance upon which the 

 great muscles of the wings are to act, thus 

 identifying this part of the body in function as 

 in structure with that of other animals. 



The Nervous System. Comparative exami- 

 nations of the nervous system in Articulata, 

 and the changes which it undergoes, more es- 

 pecially in Insects, as well as the existence in 

 it of parts which we regard as analogous to the 

 motor and sensitive portions of the spinal cord 

 in vertebrata, have invested this division of our 

 subject with more than a common amount of 

 interest. 



It has been shewn in a former part of this 

 work,f that in Articulata the most rudimentary 

 condition of the nervous system exists in the 

 form of two longitudinal cords, extended along 

 the median line of the under surface of the 

 body, parallel with each other, and nearly close 

 together, excepting at their anterior part, where 

 they diverge, and pass upwards, to embrace be- 

 tween them the commencement of the alimen- 

 tary canal. In a more advanced stage of or- 

 ganization each of these cords has a series of 

 enlargements or ganglia in its course, situated 

 at certain distances apart, and varying in num- 

 ber according to the number of segments into 

 which the body of the animal is divided. 

 These enlargements correspond precisely in si- 

 tuation in both cords, so that the nervous sys- 

 tem in this condition may be described simply 

 as composed of two knotted, parallel cords. 

 The enlargements in one cord are either placed 

 close to the corresponding ones in the other, or 

 are separated only by a very slight interspace. 

 This is the form in which the nervous system 

 exists, as we have seen, in the Talitrus,J in 

 which the ganglia are approximated together, 

 but are still distinct from each other, and form 

 a double series of enlargements, united by in- 

 tervening cords. This is also the condition in 

 which the nervous system exists in its most ru- 

 dimentary state in the larva of hexapodous 

 insects. But these parallel cords, which toge- 

 ther form the analogue of the cerebro-spinal 

 system of vertebrata, and correspond one to 

 each side of the body, are not in themselves 

 simple structures, each one being composed of 



* Bennet on the anatomy of the thorax in insects, 

 and its function during flight, Zoological Journal, 

 vol. i. p. 394. 



t Art. CRUSTACEA, ENTOZOA, ANNELIDA. 



J CRUSTACEA, vol i. p. 763, fig. 391. 



