INSECTS. 



[ 361 ] 



INSECTS. 



as before. The walls of the dorsal vessel 

 consist of longitudinal and transverse fibres, 

 surrounded externally by a very delicate pe- 

 ritoneal layer. The cavity of the heart is 

 lined by another delicate membrane, which 

 in the constricted parts forms internal val- 

 vular projections, whereby the dorsal vessel 

 is divided into as many chambers as there 

 are constrictions. Each of these cardiac 

 chambers is furnished at its front end, right 

 and left, with a fissure which can be closed 

 internally by a valvular membranous fold. 

 The cardiac chambers contract in regular 

 succession from behind forwards, and thus, 

 with the aid of the valvular apparatus, which 

 prevents the lateral exit of the blood, pro- 

 pel this liquid into the aorta. This is no- 

 thing more than a continuation of the most 

 anterior heart-chamber, and runs as a simple 

 narrow tube beneath the back of the thorax, 

 where it terminates either in a single aper- 

 ture, or divides into several short branches, 

 which also terminate suddenly in open ori- 

 fices. The number of chambers varies, but 

 very frequently there are eight. 



In the antennae, legs, and other appen- 

 dages of the body of insects, the arterial and 

 venous currents may be seen running toge- 

 ther, whilst in the wings the currents are 

 distinct. 



The blood of insects usually consists of a 

 colourless liquid containing rounded or oval, 

 colourless, nucleated corpuscles (PI. 40. fig. 

 33) ; but sometimes it is yellowish or green- 

 ish, and rarely red. 



The respiration of insects is effected by 

 means of TRACHEAE, two or more large 

 trunks of which usually traverse the body 

 longitudinally, giving off branches which run 

 in all directions, and communicating with 

 the air by numerous short tubes, connected 

 at or near the sides of the body with orifices 

 termed SPIRACLES or stigmata. Of those 

 insects which live in water, some have stig- 

 matic orifices which are brought into rela- 

 tion with the air at the surface of the water ; 

 whilst others in the larval state respire the 

 air mixed with the water in which they live, 

 this process being facilitated by the presence 

 of external branchiae, or processes of the in- 

 tegument in the form of leaves, plates, or 

 hairs, through which numerous tracheae 

 ramify in every direction. 



The nervous system of insects consists 

 essentially of a series of ganglia arranged in 

 pairs, one for each segment of the body, and 

 situated between the alimentary canal and 

 the under surface of the body. The ganglia 



of each pair are mostly united with each 

 other, but sometimes distinct, and are con- 

 nected with those adjacent by longitudinal 

 cords. The uppermost pair of ventral ganglia 

 is connected by two lateral cords surround- 

 ing the oesophagus with a large cephalic gan- 

 glion or brain. From the ganglia branches 

 are distributed to all parts of the body. A 

 sympathetic system of nerves is also present. 

 Want of space compels us to limit the 

 notice of the reproductive organs to the 

 description of PI. 27. figs. 18 & 19. 



Many insects undergo complete metamor- 

 phoses, between the period at which they are 

 hatched and that at which they attain their 

 full development. On first leaving the egg, 

 they assume a more or less worm-like form, 

 known as the larva, caterpillar, or maggot. 

 The next stage is that in which they usually 

 neither move nor take food, when they form 

 the nympha, pupa, or chrysalis. This state 

 is succeeded by that of the perfect insect 

 or imago. In other insects, however, among 

 the Orthoptera, Hemiptera, and Neuroptera, 

 the metamorphosis is incomplete, the body, 

 legs, and antennae of the larva are nearly 

 similar in form to those of the imago, but 

 the wings are wanting. In some insects, 

 also, of the above orders, the pupa con- 

 tinues to be active, differing only from the 

 larva in its larger size, and in having acquired 

 rudimentary wings (PL 28. figs. 15. 1?. 21). 

 In some insects the only change consists 

 in ecdysis, without material alteration in 

 structure. 



Examination, fyc. The external parts and 

 organs of insects are usually examined as 

 opake objects, the animals being held in 

 the stage-forceps. This method is, however, 

 often very unsatisfactory ; and the best is 

 to press them as much as possible between 

 two slides, without crushing, and to fasten 

 the slides together with india-rubber bands 

 or fine string, so that the parts may dry in 

 the compressed form. When subsequently 

 soaked in oil of turpentine, and mounted in 

 balsam, they will become much more trans- 

 parent and distinct. By prolonged macera- 

 tion in turpentine, the whole of the pigment 

 may be removed, which enables the struc- 

 ture to be seen very distinctly. When the 

 organs are very hard and thick, they may be 

 softened by boiling water, or solution of 

 potash, before being pressed between the 

 slides. 



The internal organs, which are very deli- 

 cate, must be brought to view by dissection 

 under water, the insect being fixed by pins 



