78'8 Mn. J. a, ii. frew on the larval 



Chloropa. Near the anterior venti-al border of each thoi-acic 

 segment is a transverse series of about six small rounded sense 

 organs (t.s.o.). Tiieso are similar to, but rather larger than, 

 those occurring on the ventral surface of the head, from which 

 tliey also appear to differ in being sunk in extremely shallow pits, 

 instead of being slightly raised above the surface. These sense 

 organs have not been found on the dorsal surface of the body. 

 Each thoi-acic segment also bears two little sense oigans at about 

 the middle of the ventral surface, one on each side of the median 

 line (text-fig. 2, v.l.). Each consists of three tiny papillaa situated 

 very close together and each bearing a minute stiff bristle. 

 Keilin (9) has established the fact tliat the peduncles of the 

 ima<'inal leg rudiments are attached to the hypoderm immediately 

 below these organs which he regards as vestigial legs. I have 

 been able to verify this relationship in the eucephalous larva of 

 Forcipomyia piceus Winn, where there is no peduncle to the 

 imaoinal discs, but I have failed to do so in Ghlorops, where the 

 peduncles are extremely long and fine. 



Each of the abdominal segments, except the ninth, hns a 

 broad band of chitinous teeth round its anterior border. What 

 I regard as the ninth abdominal segment is considerably smaller 

 than any of the others. Ventrally it bears a longitudinal slit, 

 the anus, bordered on each side by a slightly raised rounded lip ; 

 postero-dorsally it bears the two posterior stigmatic papillae. 

 When the larva is fully extended the division between the eighth 

 and ninth abdominal segments is indistinguishable; the ninth 

 segment is, however, capable of being partially withdrawn within 

 the eighth and should, I think, be regarded as a separate segment. 

 This conclusion is strengthened by a consideration of the distri- 

 bution of the abdominal sense organs in the larva of Meromyza 

 nignventris Mcq,, which will be dealt with liter. No abdominal 

 souse organs were found in the larva of Ghlorops. 



Tiie form of the spiracles is described later in connection with 

 the tracheal system. 



B. Internal Anatomy of Larva. 



The Cephalo-Pharyngeal Skeleton (text-figs, 3, 4 & 5, A-E) 

 consists of two mouth hooks, an intermediate sderite (i.s.) which 

 bears anteriorly an articular surface for each mouth hook, and a 

 large pharyngeal sderite (p.s.). The two latter sclerites are 

 fused, bub the line of junction is clearly visiole owing to the 

 heavier cliitinization of the intermediate sclerite. For the sake 

 of convenience the mouth hooks are, throughout this paper, 

 referred to as " mandibles," though it inust be understood that 

 no attempt is thereby made to homologize them with the man- 

 dibles of Orthorrhaphous larvae. According to Weismann's 

 embryological work (16) they are entirely new structures. JJolm- 

 gren (5) considers them to be homologous with the mandibles 

 of Orthorrhaphous larvJB but brings forward no embryological 



