58 



METAMORPHOSIS 



single pair of thoracic limbs, while the abdomen is segmented. 

 The mandibles are specially large and adapted to hold on to the 

 host-tissues. The digestive system is represented by an extensive 

 sac-like mesenteron and the proctoda^um is, as yet, hardly evident. 

 The nervous system does not exist as such and its fundaments 

 are undifferentiated from the ectoderm ; the circulatory and 

 tracheal systems are likewise undeveloped. The so-called 

 eucoiliform larva (Fig. 34) present in the Cynipid family Figitidse 

 (vide James, 1928), whose members are common parasites of 

 Dipterous larvae, is evidently a specialised 

 protopod type. Both thorax and abdomen 

 are segmented, and the limbs of the 

 first-mentioned region are often greatly 

 developed, possibly functioning as a 

 means of rupturing the enclosing em- 

 bryonic membrane. Other types of 

 protopod larvae without thoracic limbs 

 occur in the Mymaridae, Scelionidae 

 and certain other families of parasitic 

 Hymenoptera. Alongside the primitive 

 embryonic characters of protopod larvae 

 there exist also specialised adaptive 

 of Synopeas rhanis{Flaty - features. The great development of the 



Fig. 33. Protopod (cyclo- 

 poid) first instar larva 



gasteridae), lateral view 

 mm, Mandibular muscle 

 other lettering as in Fig 

 32. (From Marchal.) 



thoracic limbs in certain of the eucoili- 

 form larvae and the caudal prolongation, 

 often bifurcated in the Platygasteridae, 

 can only be interpreted as adaptional developments whose 

 functional significance is obscure. 



2. The Polypod Type. Typical examples of polypod larvae are 

 afforded by the caterpillars of the Lepidoptera and the 

 Tenthredinidae and by the very similar larvae of the scorpion-flies 

 or Panorpidae. Their essential features are the retention of 

 abdominal limbs and the presence of a peripneustic tracheal 

 system. The presence of a polypod instar in the larval develop- 

 ment of the parasitic Hymenoptera is a recent discovery. It 

 occurs among the Cynipoidea in certain genera of Figitidae (James, 

 1928) and in Ibalia, which is a parasite of Sirex (Chrystal, 1930). 



