XXX INTRODUCTION. 



transmit the faeces, they are not perfected till a late period of the larva's 

 existence. 



The head and mandibles of the larva of Trogus, so ably described by 

 George Newport, are corneous and yellow, with the margins and apices 

 of the latter black. The mandibles are curved, apically acute, fitted for 

 penetration and suction, but not for manducation. The maxillae are 

 three-jointed, with the terminal joint broad, triangular, soft and mem- 

 braneous, the second very short, and the basal joint strong and elongate. 

 The labium is triangular, with a slight median ridge. The antennae are 

 entirely rudimentary, but represented by small, corneous callosities on the 

 frons. The lateral margins of all the segments are explanate, and project 

 in the form of tubercles, which are prominent in the three thoracic seg- 

 ments, as well as in the pre-abdominal. Their tubercular form is, however, 

 most marked in the fifth and eleventh segments, which also bear distinct 

 mammaeform tubercles laterally upon the venter, acting as locomotive 

 organs. In the later stages of growth, these organs gradually more closely 

 approximate, and illustrate the greater development of the dorsal than of 

 the ventral surface. True legs are indicated only by six points upon the 

 lower surface of the thoracic segments, in the same position as the 

 abdominal tubercles. The pedal process of the apical segment is pointed 

 and projecting, and is said by Berthoumieu to be reduced to a normal 

 form after the second or third moult. It is opposable to another, beneath 

 the thirteenth segment, and with it forms a prehensile organ, by means of 

 which the larva is enabled to afifix itself and alter its position. The 

 spiracles are oval, 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. This fact abundantly disproves Boisduval's theory 

 that internal larvae do not directly breathe, but obtain sufficient oxygen 

 from their food, which, he says, is not taken through the mouth, but 

 assimilated by a kind of general absorption, in which case cribrary and 

 respiratory organs would be superfluous. Professor Owen was of opinion 

 that internal parasites undoubtedly destroy many of the minute branchial 

 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, rudiments of the eyes become 

 infumate and convex. The larval skin first splits along the cephalic 

 dorsum and in half-an-hour the pupa has entirely cast it off. Many larvae 

 of Ichneumonidae, when full-fed, leave their hosts' bodies and construct 

 for themselves silken cocoons, numbers of which are very beautiful, being 

 composed of iridescent strands or threads of mingled black and white. 

 Others, like the Ichneumoninae, remain in the chrysalis of the demolished 

 host, generally spinning for themselves an internal silken lining upon it. 



The pupa of Ichneumonidae exhibits, like that of the Coleoptera, all 

 the members of the perfect insect inclosed in individual sheaths. In 

 Ichneumon beUipes, a typical example of the first sub-family, the head is 

 deflexed upon the prosternum ; the antennae lie along the outer edge of 

 the sternum and extend to the centre of the fourth ventral segment ; the 

 mandibles are closed and the palpi extended straight downwards until their 



