PAETS OF THE BODY 993 



its extremity, with a double row of small projecting barrel-shaped 

 1 ><><lies (shown in fig. 740), which are surmised by Mr. Newport 

 (whose opinion is confirmed by the kindred inquiries of Dr. Hicks) 

 to be organs of taste. Numerous other modifications of the structure 

 of the mouth, existing in the different tribes of insects, are well 

 worthy of the careful study of the microscopist ; but as detailed 

 descriptions of most of these will be found in every systematic trea- 

 tise on entomology, the foregoing general account of the principal 

 types must suffice. 



Parts of the Body. The conformation of the several divisions of 

 the alimentary canal presents such a multitude of diversities, not 

 only in different tribes of insects, but in different states of the same 

 individual, that it would be utterly vain to attempt here to give 

 even a general idea of it, more especially as it is a subject of far 

 less interest to the ordinary microscopist than to the professed 

 anatomist. Hence we shall only stop to mention that the 'muscular 

 gizzard,' in which the oesophagus very commonly terminates, is often 

 lined by several rows of strong horny teeth for the reduction of the 

 food, which furnish very beautiful objects, especially for the bino- 

 cular. These are particularly developed among the grasshoppers, 

 crickets, and locusts, the nature of whose food causes them to require 

 powerful instruments for its reduction. 1 



The circulation of blood may be distinctly watched in many of 

 the more transparent larva?, and may sometimes be observed in the 

 perfect insect. It is kept up by a ' dorsal vessel ' (so named from 

 the position it always occupies along the middle of the back in the 

 thoracic and abdominal regions), which really consists of a succession 

 of muscular contractile cavities, one for each segment, opening one 

 into another from behind forwards, so as to form a continuous trunk 

 divided by valvular partitions. In many larvae, however, these 

 partitions are very indistinct ; and the walls of the ' dorsal -vessel 

 are so thin and transparent that it can with difficulty be made out. 

 a limitation of the light by the diaphragm being often necessary. 

 The blood which moves through this trunk, and which is distributed 

 by it to the body, is a transparent and nearly colourless fluid, carry- 

 ing with it a number of 'oat-shaped' corpuscles, by the motion of 

 which its flow can lie followed. 2 The current enters the 'dorsal 

 vessel' at its posterior extremity, and is propelled forwards by the 

 ci infractions of the successive chambers, being prevented from moving 

 in the opposite direction by the valves between the chambers, which 

 only open forwards. Arrived at the anterior extremity of the 

 dorsal vessel,' the blood is distributed in three principal channels : 

 a central one, namely, passing to the head, and a lateral one to either 

 side, descending so as to approach the lower surface of the body. 

 It is from the two lateral currents that the secondary streams 

 diverge, which pass into the legs and wings, and then return back 

 to the main stream ; and it is from these also that in the larva 



1 The student who desires to carry further the study of the digestive ;q>|niratus 

 should consult Professor Plateau's memoir, ' Recherches sur les Phenomenes de 

 la Digestion chez les Insectes,' .!/< '///. Ai-inl. lioij. tie JJrlyiqiu; xli. 



2 On the blood-tissue of insects consult Mr. W. M. Wheeler in vol. vi. of the 

 American journal Psyche. 



:-i s 



