INSECTS. 



or wanting, M. Lamarck considers this supposition as probably 

 the correct one. The organs of touch have been generally con- 

 ceived to be those named antenna or feelers ; and insects de- 

 stitute of these use their palpi and the tarsi of the anterior feet 

 for the same purpose. The covering of the body being gene- 

 rally corneous, can communicate but feebly the sense of touch. 

 In Insects distinct absorbing or circulating vessels have not 

 hitherto been discovered. A dorsal vessel, or long transpa- 

 rent canal reaches indeed from the head to the posterior ex- 

 tremity of the body ; and this has been conceived to be equiva- 

 lent to the heart and blood-vessels of the higher classes. But 

 this vessel, though narrowed at intervals, corresponding to the 

 segments of the body, and having an undulatory contraction 

 and dilatation from the head to the posterior extremity, pos- 

 sesses none of the characters of a true heart or circulating 

 system ; and it is considered to be only the chief reservoir of 

 the principal fluid in insects, filling and emptying itself by ab- 

 sorption and exudation. M. Cams, however, has discovered in 

 the caudal laminae of some larvae, and in the rudimentary wings, 

 an excurrent and incurrent motion of fluid in distinct tubes, which 

 he conceives to be a true circulation ; a circumstance which 

 had escaped the penetration of Lyonnet, who always found the 

 undulatory motion of the dorsal vessel to proceed invariably from 

 the head to the tail. As this circulation, or double motion, 

 however, has only been observed in one stage of existence, it 

 is rather to be regarded as a phenomenon connected with the pas- 

 sage of the animal into a different state, than as a circulation 

 analogous to that of animals with two distinct sets of vessels. 



Respiration in Insects is effected by means of two tubes, 

 one on each side of the body, and running along its whole 

 length, named trachecc. From these tracheal vessels are de- 

 rived a great many ramifications or bronchi, the number of which, 

 is more or less considerable as they belong to parts enjoying 

 more or less vital energy. The tracheae communicate with the 

 external air by means of openings called stigmata, of which 

 the number varies, placed on each side of the body. In cater- 

 pillars the number of stigmata is generally eighteen. These 

 stigmata are marked in the sfein of the insect by a small scaly 

 plate, open in the centre, and furnished with membranes or fila- 

 ments to protect the entrance. The larvae of many species 



VOL. II. P 



