FORMATION OF THE IMAGO OF CORE THE A 



669 



pound eyes, and this is a rare exception among the Holometabola, 

 are present in the larva. On the other hand, the thoracic legs, the 

 wings, and halteres are developed out of new rudiments which are 

 present in the last larval stage, before 

 pupation. Each thoracic segment has 

 four of them, two ventral and two 

 dorsal (Fig. 622) ; the ventral buds 

 becoming the legs. Of the dorsal 

 pairs, that of the mesothorax develops 

 into wings, that of the metathorax 

 into halteres, while from the corre- 

 sponding rudiments of the prothorax 

 in Corethra arise the stigma-bearing 

 dorsal or respiratory processes of the 

 pupa, and in Simulium a tuft of 

 tracheal gills (Fig. 623, ra ; see also 

 Fig. 582). 



These imaginal buds may be re- 

 garded as evaginations of the outer 

 surface of the body. The only differ- 

 ence is that the buds of the append- 

 ages as a "whole seem sunken below 

 the level of the surface of the body, 



being situated at the bottom of an evagination, as in the buds of 

 the head and trunk in the Pilidium larva of nemertean worms, and 

 in the rudiments of the lower surface of 

 the body of Echinus present in the pluteus 

 larva. 



The lumen of the invagination in which 

 the appendages of Corethra (and other Holo- 

 metabola) are situated is called by Van 

 Rees the peripodal cavity, and the external 

 sheath bordering it, which is naturally con- 

 tinuous with the hypodermis of the body, 

 FIG. 624. -imaginal buds in the P^ipodal membrane (Fig. 636, p). 



larva of Corethra (diagrammatic 



cross-section of thorax): invairi- , TT , ,, ,, , ., ,. f 



nations (fe and be) of the larval We must adopt the view that the rudiments of 

 hypodermis (ihy), in whose bases the appendages (imaginal buds) are from the first 



the rudiments of wings ( fa) and ,..,,. , 



legs (ba) arise ; ih, chitinous in- divided into ectodermal and mesodermal portions, 

 tegument of the larva. After w hich are derived from the corresponding germ- 

 layers of the larva. The ectoderm of the rudi- 

 ments of the appendages is continuous with the peripodal membrane, and 

 through it with the hypodermis. Weismann was inclined to derive the organs 

 (tracheae, muscles, etc.) developing within the germs of the appendages from a 



FIG. 623. Late larva of Simulium, 

 showing 1 the rudiments of the pupal 

 structures within the larval skin : i l , /'-', 

 I 3 , fore, middle, and hind legs of the fly ; 

 ra, respiratory appendages of pupa ; 

 ic, wing of fly ; h, halter of fly. After 

 MialL 



