II.] METAMORPHOSES OF INSECTS. 37 



indicated only by the arrangement of the muscles. 

 slkf is the oesophagus leading into a wide stomach 

 which occupies nearly the whole body, gsae is the 

 rudiment of the supra-cesophageal ganglia, bsm the 

 ventral nervous cords. The ventral nervous mass has 

 the form of a broad band, with straight sides ; it 

 consists of embryonal cells, and remains in this un- 

 developed condition during the whole larval state. 



At the next moult the larva enters its third state, 

 which, as far as the external form (Fig. 27) is con- 

 cerned, differs from the second only in being some- 

 what more elongated. The internal organs, however, 

 are much more complex and complete. The tracheae 

 have made their appearance, and the mouth is pro- 

 vided with a pair of mandibles. From this point 

 the metamorphoses of Platygaster do not appear to 

 differ materially from those of other parasitic Hymen- 

 optera. 



An allied genus, Polynema, has also very curious 

 larvae. The perfect insect is aquatic in its habits, 

 swimming by means of its wings ; flying, if we may 

 say so, under water. 1 It lays its eggs inside those 

 of Dragon-flies ; and the embryo, as shown in 

 Fig. 28, has the form of a bottle-shaped mass of undif- 

 ferentiated embryonal cells, covered by a thin cuticle, 

 but without any trace of further organization. Pro- 

 tected by the egg-shell of the Dragon-fly, and bathed 

 in the nourishing fluid of the Dragon-fly's egg, the 

 young Polynema imbibes nourishment through its 

 whole surface, and increases rapidly in size. The 

 digestive canal gradually makes its appearance ; the 



1 Transactions of the Linnean Society, 1863. 



