302 J. W. SCOTT MAC FIE. 



comb. The general form remains nevertheless fairly constant in each phase, and the 

 most conspicuous development observed to take place was a lengthening and general 

 enlargement of one of the central barbs until it became an outstanding feature. 



Siphon (figs. 6, 7). In four larvae belonging to the four phases the lengths and 

 l)readths (at the base) of the siphons were respectively 12 by 5, 19 by 8, 37 by 16, and 

 46 by 30 units. In the first three phases therefore the length of the siphon was rather 

 over twice its basal diameter, but in the last phase it was considerably less than this. 

 In the last phase practically the whole of the siphon is chitinised and is therefore little 

 liable to distortion. The averages of length and breadth of the siphon in ten such 

 larvae taken at random were respectively 49*4 and 29*9 units, a proportion equal to 

 1-65 to 1. 



It is generally stated that the length of the siphon of Stegomyiafasciata is more than 

 twice its breadth, and this character has been made use of for the identification of 

 this species in the larval stage. It is important to realise that this statement is not 

 strictly accurate in the case of larvae in the last phase of their development, and in 

 some individuals in their earlier phases also. 



It is characteristic of Stegomyiafasciata that the young larva has only the tip of the 

 siphon dark-coloured. The dark part of the siphon is of course the chitinised part, 

 and this increases in proportion as the larva matures. The larva immediately after 

 emerging from the egg has no part of the siphon dark, but during the first phase the 

 tip becomes chitinised for from 4 to 6 units of its length. The chitinised part measures 



9 to 13 units in the second phase, 25 to 31 in the third, and 43 to 50, or practically the 

 whole length, in the fourth phase. 



Pecten. The pecten consists of a single row of scales running nearly parallel to the 

 sides of the siphon. The last scale is sometimes detached from the others, and some- 

 times there is a scale situated to one side of the general rank. 



The number of scales is variable both in the same individual on its two sides, and 

 in different larvae at the same stage of development, but it increases rapidly with 

 successive moults. In the first phase there are usually 4 to 5 scales, in the second 

 8 to 10, in the third 12 to 18, and in the fourth 14 to 20. In some individuals the 

 number of scales increases progressively, for example the successive pelts cast by one 

 larva showed respectively 4, 8, 16, and 19 scales ; in others the maximum number 

 may be reached in the third phase as occurred in a larva which showed in the four 

 phases 5-5, 9-8, 18-15, and 18-15 pecten scales. Each scale has a variable number 

 of barbs and, as in the case of the comb, the most notable changes observed during 

 development are the strengthening of the scales, and the differentiation of one barb 

 into a long and conspicuous process (fig. 5). 



Hair-tuft on the Siphon (figs. 6, 7). The hair-tuft on the siphon is usually situated 

 just beyond the last scale of the pecten, that is in the first phase 6 to 11, in the second 



10 to 18, in the third 22 to 25, and in the fourth 25 to 31 units above the base, but 

 sometimes it lies on a level with one of the last scales. It consists of a single simple 

 hair in the first phase, usually a double but sometimes a triple hair in the second 

 phase, a triple hair in the third phase, and a hair with 3 to 5 branches in the fourth 

 phase. The position of the hair-tuft is in every phase a little beyond the middle of 



