28 BRITISH BUTTERFLIES. 



into the pericardial cavity, and from thence re-enters the dorsal 

 vessel again. 



(4) The respiratory system : The air is conveyed into all parts of the 

 body by means of the tracheae, elastic tubes, held open by an inner 

 chitinous layer, which are all intimately connected. Large tubes 

 connect the spiracles longitudinally, others pass from one side of the 

 body to the other, whilst a set of tracheae in the lower part of the body, 

 is connected with another set in the upper part by ascending tubes. 

 These main branches give out small branches, which fork in all 

 directions, and by them the body is supplied plentifully with air. The 

 tubes have a white glistening appearance, and hence can be readily 

 detected in a freshly killed insect without difficulty. The finest 

 tracheal tubes are supposed to penetrate cells, but it rs not known 

 whether they terminate with open or closed extremities. 



(5) The fat-body : The fat- body is a very prominent part of the 

 structure of lepidopterous larvae. It consists of fat masses of various 

 sizes and colours, loosely connected together, and enveloping most of 

 the organs. It varies in colour and appearance in almost every species, 

 and appears to consist essentially of a reservoir, as it were, of reserve 

 material, which increases in the larval stage, when feeding is going on 

 rapidly, and upon which the insect can draw in the future, when it is 

 unable for a long period to take food, e.g., at the exuviation of each 

 larval skin, and the more exhausting periods of metamorphosis. It must 

 also be looked upon as material which the insect can utilise, during 

 the period of histogenesis in the pupal stage, in the formation of the 

 imaginal structures. Bessels notes that in Pieris brassicae, the fat-body 

 is white. Jackson, however, observes that, in P. brassicae, the fresh fat- 

 body posteriorly to the 6th segment is greenish or olive-yellow, 

 anteriorly to it, opaque yellow or green on the dorsal aspect, but on the 

 ventral aspect, white. He also says that the fat-body of the larva of 

 Vanessa io is yellow, and that it becomes orange in the pupa. 



(6) The nervous system: The nervous system of the caterpillar is, in its 

 broad outline, not very dissimilar from that of the butterfly and is very 

 interesting, and its structure helps to explain why it is that, when the 

 thorax of the butterfly is crushed and the insect is, to all appearances, 

 dead, the abdomen, head and antennas continue to twitch and move, 

 and give to the kindhearted, but ignorant, observer, the notion that 

 one's cruelty is unbounded in pinning an insect alive to suffer tortures 

 through being spit on a pin when in a moribund condition. The fact is, 

 the central nervous system of an insect is apparently very different 

 from that of vertebrate animals, and is situated in the ventral or belly 

 part of the body, not, as in the latter, in the dorsal. In each of the 

 abdominal and thoracic segments there are two ganglia (little masses of 

 nervous tissue) placed one on either side of the central line. In the head, 

 which appears to be composed of four segments, the eight ganglia are 

 massed together around the oesophagus. Each ganglion is united 

 to its fellow in the same segment by minute transverse nerve fibres, 

 whilst other fibres pass from it in a longitudinal direction to the ganglia 

 of the same side, next in front of and behind it. In addition, the ganglia 

 of the thorax and abdomen give origin to numerous nerves which are 

 distributed to the organs of alimentation and circulation, and to the 

 muscles, those from the thoracic ganglia being chiefly distributed to 

 the muscles that move the wings (when these organs are present). 



