348 



of the accessary copulatory organs of the ninth segment, or 

 in the modifications of the segments to aid in the eversion of • 

 the penis. In the larva shortly before pupation, the ninth 

 segment develops terminally a pair of great ''beak-like" clasp- 

 ing forceps, which have a very important accessory copulatory 

 function, while the sternal regions of the seventh and eighth 

 segments, which have not kept pace with the extension of 

 the t-ergal region of these segments, become pushed forwards 

 and are partly invaginated as the penis develops (fig. 26). 



Shortly after the penis adopts its adult proportions 

 muscles become developed within it. 



The histological processes underlying the changes will be 

 dealt with later in connection with the development of the 

 integument. 



The general blackening of the cuticle of the Dupa com- 

 mences some three and a half days (in summer) aft-er the last 

 larval moult, and is complete about twelve hours later; the 

 wasp remains enclosed in the pupal sheath for twelve to 

 twenty-four hours longer, and then, splitting the thin sheath 

 which imprisons it, escapes. 



B. — The Integument (Histological Development). 



In the newly hatched larva the integument has reached 

 a state of development, which it retains with but small changes 

 throughout the feeding period. The ectoderm consists, for its 

 greater part, of a single layer of cells which are of two kinds; 

 there are the large cells, less numerous than the other type, 

 but occupying a greater part of the integument — the true 

 "larval-cells;" and, secondly, there are the narrow strips of 

 integument consisting entirely of smaller more embryonic 

 cells — the centres from which the imago will later develop — 

 the imaginal discs of the integument; indeed, at this early 

 period the rudiments of the wings, legs, mouth appendages, 

 antennae, and even of the eyes are clearly recognizable, while 

 the areas from which the general body surface of the imago 

 will later develop are very prominent. 



It is to the development of the general body integument 

 that we will first give our attention ; this will be followed by 

 the description of the formation of the legs, wings, antennae, 

 and mouth appendages; and, finally, the most astonishing of 

 all the integumental changes, the development of the 

 compound eyes and ocelli will be described. 



In the newly hatched larva the cells of the imaginal discs 

 of the general body surface are small, short, and columnar, 

 and closely packed side by side (fig. 10); their protoplasm is 

 clear, and their nuclei are very large. As the larva grows 



