INSECTA. 



965 



size of different parts of the nervous system is 

 effected is in full accordance with the recently 

 developed facts of Ehrenberg and others re- 

 specting the tubular nature of the primary 

 nervous fibres. 



At forty-eight hours, (fig. 423, 9) the whole of 

 the cords have regained the longitudinal direc- 

 tion, and the seventh ganglion, which had begun 

 to be decreased in size at the last period, has 

 now also entirely disappeared, and its nerves, 

 like those of the fifth and sixth, come from the 

 cord, while the ganglia of the thorax are ac- 

 quiring a great size. 



Fig. 423. 



9 10 



Fo-rty-eiyht hours. Fifty-eight hours. 



At fifty-eight hours (10) a further change 

 has been effected. The second and third tho- 

 racic ganglia have united, and the double gan- 

 glion thus formed is only separated from the 

 larger thoracic mass composed of the fourth, 

 fifth, and part of the sixth ganglia, by the short 

 but greatly enlarged cords which pass, as before 

 noticed, on each side of the central attachment 

 of the muscles. The transverse plexus are 

 united with the nerves to the wings, and the 

 whole of these gangliated portions of cord have 

 been carried forwards, and now occupy the 

 middle portion of the immensely enlarged 

 meso-thorax. The optic and antennal nerves 

 have nearly attained their full development, 

 and those numerous and most intricate plexus 

 of nerves in the three thoracic segments of 

 the larva form only a few trunks, which can 

 hardly be recognized as the same structures. 

 The arrangement of the whole nervous system 

 is now nearly as it exists in the perfect insect. 

 The whole of these important changes are thus 

 seen to take place within the first three days 

 after the insect has undergone its metamor- 

 phosis; and they precede those of the alimentary 

 canal, generative system, and other organs, 

 which are still very far from being completed, 

 and indeed, as compared with the nervous sys- 

 tem, have made but little progress. 



Such is the rapidity of these changes, as ob- 

 served by us in June, 1832, in a species that 

 usually undergoes its metamorphosis from the 

 larva to the perfect state in about fourteen days. 

 On repeating our observation on the same insect 

 in the following August, when, from the in- 

 creased temperature of the season, the whole of 



the changes in the, body were completed in 

 about eight days, we did not observe that these 

 had become much accelerated, although the 

 changes in the other structures were hastened. 

 The whole of these phenomena are induced by 

 an alteration which takes place in the external 

 tegument, and the permanent contraction of 

 the longitudinal and diagonal muscles of the 

 body, by means of which the anterior margin 

 of one segment is drawn beneath the posterior 

 of that which immediately precedes it. This 

 is carried to a greater or less extent in the diffe- 

 rent segments, and the nervous cord being in 

 consequence rendered too long to lie in a direct 

 line, a disposition is thus induced in its va- 

 rious parts to coalesce. 



Organs of Nutrition. The chief organs of 

 nutrition in insects, the alimentary canal and 

 its appendages, assume a variety of forms in 

 the different classes, and undergo changes al- 

 most as remarkable as those of the nervous 

 system. From being scarcely more than a 

 simple elongated tube, with a few slight en- 

 largements in its course, as in some of the 

 apodal Hymenoptera, the alimentary canal be- 

 comes in the perfect individual a long convo- 

 luted organ, thick, muscular, and divided into 

 several compartments, each of which is adapted 

 to a peculiar function, but subservient to the 

 more general one of assimilating the food re- 

 ceived, into one homogeneous material, fitted 

 for the nourishment of the whole body. But 

 whatever be its particular form, the alimentary 

 canal may be regarded as composed of three 

 distinct coats or tissues, which we shall dis- 

 tinguish as the external or peritoneal, the middle 

 or muscular, and the internal or mucous. 



The pe.ritonaal coat, or layer, is an exceed- 

 ingly transparent, white, shining, and delicate 

 membrane, and is observed only with great 

 difficulty. It covers the outer surface of the 

 muscular coat throughout the whole course of 

 the canal, and, as we are strongly induced to 

 believe, although we have not positively ascer- 

 tained it, is continuous with and reflected 

 along the tracheal vessels that ramify on the 

 stomach, -and forms their external covering. 

 \Ve have never been able to detach it from the 

 muscular coat, which it completely invests, 

 and to which it closely adheres, but we have 

 seen it most distinctly in recently killed insects 

 more particularly in the Apidte, as in Antho- 

 phora retusa, when the canal has been removed 

 from the body and viewed by transmitted light. 

 It is then seen most distinctly extending along 

 the sides of the canal, directly across the 

 angles formed by the contraction of some part 

 of the muscular coat, where this is thrown into 

 folds or depressions. 



The muscular, or middle coat, is very strongly 

 marked. It is composed of transverse and longi- 

 tudinal fibres, interlaced with each other, and also 

 of a series of oblique fibres, which, as shewn by 

 Lyonet in the Cossus, sometimes in part form the 

 retractoresventriculi muscles, that assist to retain 

 the canal in its proper position in the body, and 

 connect it with the whole muscular system. 

 Burmeister states,* that distinct transverse and 



* Op. cit. (trans,) p. 121. 



