329 



following three segments gives the thorax the general shape 

 that it has in the imago; the abdomen shortens considerably, 

 rounds itself off, constricts considerably at both ends, while 

 at the same time portions of its anterior segments move for- 

 wards, and fuse with the thorax to form a compound struc- 

 ture, the ''alitrunk," so characteristic of the Hymenoptera. 

 The head and its appendages may be considered first. 



The Head and its Appendages. 



The head of the adult wasp is developed from the first 

 two segments of the larva; the first segment, to which the 

 name oral segment may be applied, develops into the front 

 and lower portions of the head, and gives rise also to the 

 antennae and lab rum. The second segment may be called 

 the post-oral segment; from it develop the upper and occipital 

 regions of the head, including the ocelli and great eyes, while 

 below it produces the maxillae and labium and also the 

 mandibles. 



The fact that the first two segments of the larva are 

 concerned in the formation of the head can readily be verified 

 by following the spiracle of the third segment through the 

 metamorphosis, the spiracle remaining as that of the first 

 thoracic segment. Already in the late feeding period of the 

 larva, the imaginal discs of the head appendages have become 

 clearly visible. From the upper portion of the first head 

 segment the antennae grow out as short thick processes, 

 which, on account of the pressure of the larval cuticle above 

 them, are forced to grow downwards (figs. 3, 12). Each 

 antenna has, towards its distal end, a short blunt papilla, 

 which fits into the sensory structure on the first segment, 

 referred to above. 



Around the mouth, the other head appendages soon 

 become prominent; immediat-ely in front of the mouth are 

 a pair of quite distinct outgrowths — the rudiments of the 

 labrum — which structure is, at this stage, distinctly paired 

 (fig. 13). The labrum is generally regarded as a simple, 

 unpaired downgrowth from the upper edge of the mouth, but 

 in Nasonia its paired condition is quite clear; Patten has 

 also figured the labrum as a paired structure in Acilius. (See 

 Korschelt and Heider, part iii., p. 326, fig. 160). 



The other mouth appendages are developed from the 

 second (post-oral) segment; their actual interpretation is, at 

 first sight, very confusing, for though they are developed 

 from the post-oral segment, some of them take up a position 

 actually somew^hat in front of the mouth, which is situated 

 well within the first segment. The apparent paradox finds 

 its explanation in two facts : firstly, the small mandibles of 



