333 



probably as an abnormality (since some individuals do not 

 appear to show it) ; the occurrence of this palp definitely 

 proves the homology of the mandible with a metameric 

 appendage. 



The curious nature of the labium is worthy of special 

 attention. Its anterior surface, formed as a chitinisation of 

 the protoplasmic ''pubescence" already described, is developed 

 into a strong rasping-organ. Though present in both sexes 

 the "rasp" is more strongly developed in the female. The 

 strength and efl&eiency of the labium is further increased by a 

 pair of hard, outwardly diverging chitinous bars lying within 

 its distal segment. 



Also to be especially noted is the fact that the head 

 appendages first grow in size, and not till the mature size has 

 been reached, and even exceeded, does differentiation take 

 place. This same fact will be seen also in the development 

 of the legs and ovipositor and other appendages; it seems, 

 indeed, to be true of the body surface in general (fig. 8) : 

 first the body becomes moulded, then it begins to undergo 

 differentiation producing the various joints, spines, bristles, 

 sculpturings, etc., that adorn the insect's body — first growth 

 and arrangement, then differentiation. This fact can be 

 demonstrated especially clearly in the compound eyes (see 

 these). The post-defaecation and resting periods are the time 

 in which the optic cells adopt their arrangement; in the pupa 

 they differentiate. 



Looked at in this light it is possible to regard the pupa 

 not merely as an artificially conceived, but as an embryo- 

 logically quite distinct phase. Growth occurs in the resting 

 and post-defaecation periods; the pupal period is the period 

 of differentiation. It should be clearly understood that these 

 remarks refer merely to the external characters (integument). 



Meanwhile the great eyes and the ocelli have been devel- 

 oping. These structures are merely modifications of the 

 integument; already in the resting stage the great eyes are 

 clearly recognizable; they grow over a large part of the sides 

 of the second segment. In the defaecating larva facets are 

 already clearly indicated; these become more distinct as 

 development continues. At the end of the larval life the 

 eyes are very large, and have assumed their typical bulged 

 appearance. After thirty-six to forty hours the eyes gradually 

 change from a creamy to a pale-reddish colour, which becomes 

 bright red a day later. 



The ocelli have meantime been developing at the vertex 

 of the head, and are seen in the newly formed pupa as three 

 prominent rounded tubercles arranged in a triangle. 



After about three and a half days the head gradually 



