366 INSECTA. 



lie as rudiments in a ventrally placed germ-band, and only attain development 

 in the next stage. Into this latter the Cyclops-stage passes through a moult, 

 and the larva is now an oval limbless body without segments (Fig. 182, 2G). 

 The nervous system, the salivary glands, and the proctodaeum now form as 

 ectodermal invaginations, and the groups of muscles, by the arrangement of 

 which the segmentation is recognisable, gradually develop. The last (third) 

 larval stage, which follows the preceding after a moult, has the form of a 

 segmented maggot devoid of limbs (27). 



The larval forms in other related genera seem to vary greatly. In Tcleas 

 there is also a Cycloj)s-stage, but it is preceded by a spindle-shaped larva which 

 is more ecpially segmented and has small stump-like mouth-parts, while still 

 devoid of grasping hooks (Ayers). Development here begins with the formation 

 of a coeloblastula (Metschxikoff, Ayeks), in the inner cavity of which a 

 lower layer forms by the immigration of cells. The rise of a median groove 

 marks the bilateral symmetry of the embryo and an anterior thickening dis- 

 tinguishes the cephalic end. 



All these larval forms of the Pteromalidac must be regarded as highly 

 specialised, but we are not in a position to determine in individual cases the 

 ontogenetic significance of the development of these remarkable forms.* 



The larval forms of the Insecta are very varied. A comparative 

 review of them shows most clearly that the manner of life of the 

 larva is the chief factor in determining its outward appearance. 

 "We thus have, in the phytophagous larvae that feed on leaves, the 

 eruciform type or polvpod caterpillar, in the forms that bore into 

 wood, a similar type with powerful mouth-parts and strong cephalic 

 capsule, but with degenerate limbs ; where the life of the larva is 

 more or less parasitic, the form is that of a maggot, etc. In other 

 groups (Orthoptera genuina), the larvae of which agree in their 

 manner of life with the adult, the outward appearance of the 

 imagines is to a large extent already found in the larval forms. 

 It is evident from these considerations that the metamorphosis of 

 the Insecta can only to a limited extent be utilised for phylogenetic 

 purposes. 



Above all, we must bear in mind that the larva which comes from 



the egg already shows the typical segmentation of the body, and that, 



therefore, in no single case are the ancestral forms which preceded 



the oldest Insect forms reproduced in the larvae. All that we can 



learn from the larvae of the Insecta is of value merely within the 



limits of this class. 



* [Kulagix (No. XXVIII.) has recently reinvestigated the development of Pla- 

 tygaster with special regard to the origin of the germ-layers ; there is no yolk, and 

 the total cleavage which occurs is regarded by this author as a modification of 

 superficial cleavage. Henneguy (No. XII.) similarly finds total cleavage and 

 one embryonic envelope in the nearly allied Clialcididae. A curious condition 

 is found by Marchal (No. XXXIII.) in Enajrtus fuscicollis, a form closely related 

 to I'latygastcr ; here the ovum gives rise, not to one egg, but to a legion of 

 small morulae, which form chains of 50-100. — Ed.] 



