MIGRATORY LARVJE 



333 



but it appears that having left the gut the larva bores through 

 the neck region of the host in order to establish respiratory 

 connection with the exterior. In this position it becomes 

 enveloped in an integumentary sheath and, when nearing maturity, 

 the sheath is ruptured and the larva becomes free in its host, 

 mainly feeding upon the fat-body. 



2. Types of Migratory Primary Larvae. In many cases where 

 the parasitic insect deposits 

 her eggs or larvae on leaves, 

 buds, in soil or other situa- 

 tions, but not actually on the 

 host itself, the primary larvae 

 assume a form very different 

 from that found in subsequent 

 instars, and hypermetamor- 

 phosis results. Under such 

 conditions the young larvae 

 are active migratory organisms 

 well adapted for the business 

 of seeking their hosts. This 

 kind of behaviour has been 

 independently acquired among 

 certain members of no less than 

 five orders of insects, which 

 will be discussed separately. 



(a) In the Chalcid family, 

 Perilampidae, the first stage 

 larvae of a number of species 

 of Perilampus are minute, 

 elongate, darkly coloured objects about 0-1 to 0-16 mm. long. 

 The dorsal integument is heavily . sclerotised, there is a well- 

 defined head followed by twelve body-segments and a pair of 

 cerciform caudal setae ; ventrally the body is provided with an 

 armature of spines and spinous processes which aid in locomotion. 

 Such a larva is termed a planidium (Fig. 77). H. S. Smith (1912) 

 has shown that the planidium of Perilampus hyalinus bores into 

 caterpillars of Hyphantria textor, of which it is a hyperparasite. 



Fig. 77. Planidium of Perilampus. «, 

 Ventral ; 6, dorsal. (After H. S. Smith.) 



