METAMORPHOSES. 7 



means of which the larva is enabled to affix itself and alter its 

 position. The spiracles are oval and corneous, and situated on 

 either side of the anterior part of each segment. The whole 

 of the tracheal vessels are distinct and distended with air; and 

 this fact abundantly disproves BoisduvaPs theory that internal 

 larvae do not directly breathe, but obtain sufficient oxygen from 

 their food, which, he thought, was not taken through the mouth 

 but assimilated by a kind of general absorption, in which case a 

 sucking-mouth and respiratory organs would be superfluous. 

 Professor Owen was of opinion that internal parasites undoubtedly 

 destroy many of the minute tracheae which ramify in the adipose 

 tissue, and these would probably permit the escape of sufficient 

 air for the respiration of the parasitic larvae. 



"When full-fed, the larva emits rejectamenta to the extent of 

 half its former size, and the abdominal segments become much 

 shrivelled, though the thoracic are inflated and distended. About 

 a month after evacuation the larva begins to assume the pupal 

 state. The head is then swollen and retracted into the thoracic 

 segments, and the rudiments of the eyes become darkened and 

 convex ; the larval skin first splits along the back of the head, 

 and in half an hour the pupa has totally thrown it off. Many 

 Ichneumonidous larvae when full-fed leave their host's body and 

 construct for themselves silken cocoons, numbers of which, and 

 especially in the OPHIONIN. ; E, are very beautiful, being composed 

 of iridescent stands or threads of mingled black and white, grey, 

 and sometimes red or coppery hue. Others, like the ICHNEU- 

 MOXINJE, remain in the chrysalis of the demolished host, generally 

 spinning for themselves, like Pimpla instigator, an internal silken 

 lining upon it. 



The pupae of ICHNEUMONID.E exhibit, like those of beetles, all 

 the members of the perfect insect inclosed in individual sheaths : 

 the head is deflexed upon the prosternum, the antennae lie along 

 the outer edge of the sternum and extend to the centre of the 

 abdomen or further, according to their length ; the mandibles are 

 closed and the palpi extended straight downwards until their 

 apices nearly touch. The abdomen is fully developed, though the 

 lateral tubercles are still very conspicuous, and the conformation 

 of the basal segment, which is now pressed closely upon the meta- 

 notum, undergoes considerable modification during the pupal 

 state. The genital organs are strongly protruded, at first rudi- 

 mentary and surrounded by a dense liquid, but gradually becoming 

 perfect as emergence approaches. The legs are pressed closely 

 to the sides of the body ; the femora drawn forward and the 

 tibiae folded back upon them ; the tarsi lie along the outer side 

 of the antennae in a continuous line, the hind ones extending to 

 near the abdominal apex. 



As is the case with Hymenoptera in general, the length of the 

 final quiescent condition of the larva is inversely proportionate to 

 the brevity of its pupal existence, which often is no longer than 

 of ten days' duration ; but after the imago has thrown off the 



