428 



.STUDIES IN LIFK-m.STORIRS OP AUSTRALIAN DIPTF.RA ERACHYCERA, 



The whole jaw is supported by its base on a central chitinous peg, which 

 acts as a pivot on which the jaw can move with a rocking movement. The 

 muscles which move it are shown in Text-fig. 13. They extend back through 

 the head for some distance, to be inserted posteriorly in the head wall. Average 

 measurements for the jaw axe 0.088 mm. from apex to base, and 0.085 mm. in 

 diameter from front to back. 



The pivots on which the jaws work form part of the chitiuous wall sur- 

 rounding the anterior end of the pharynx. The oral aperture is situated just 

 at the base of the median process, where the latter opens out into the internal 

 cavity of the head (.Plate xxxiii., fig. la) and here the strongly chitinised rim 

 of the pharynx grows up on each side, in the form of a stout, curved, pointed 

 pillar (Text-fig. 12) to support the jaw. Lying free between these, in the 

 oral o])ening, are two verj- small ))locks of ehitin. The opening is quite con- 

 cealed by the overlying jaws and (irocesses, and is only revealed by careful 

 dissection. 



In his description ol' the mouth parts of Stratiorayid larvae which 

 he had examined, Becker makes the statement that there is no median mouth 

 opening, and that nourisliment is taken in through the lateral slits in the walls 



Text-figures 14-17. 



Drawn from cast skins of first ecdysis. 



14. Front view of head, showing terminal mouth, and jaws, (x 320). 15. General view 



of head, from antero-dorsal aspect, (x 190). Iti. Anterior part of head, dorsal view. 



(x320). 17. Internal view of same, showing pharynx, (x 320). 



(^.r. , dorsal ridge ; i/.i., dorsal sclerite ; >«., mouth; i/i., spiracle. 



of the median process which, he suggests, has been formed by tiie coalescence 

 of upper and lower lips. That this is not the case with the larva of Metoponia 

 rub rice ps is very clearly shown by an examination of cast larval skins, found 

 with the empty egg cases and newly hatched larvae. These skins have been 

 described in the second paper of this series (Irwin-Smith, 1921). The east of 

 the head retains tiie original shape and position of all the parts, and is very 

 interesting as showing an earlier stage of development than is seen in the 

 hatched larva itself. In the cast skin tiie mouth opening is (juite terminal in 

 position, and is bounded only by the two stout jaws (Text-figs. 14-17). Of the 

 lateral ehitin plates, which afterwards ensheath them, there is here no trace. 

 'I'iie dorsal sderite is already well develojied and clearly marked off by a 

 thickened rim {d.s.). In the middle of the sclerite there is a longitudinal ridge 

 (d.r.) whicii terminates towards the anterior end in a short, pointed outgrowth. 

 Tliis is evidently the rudiment of the median proc^ess. A careful examination of 

 the position of the hairs surrounding it seems to show that the median process 

 of the fnlly-develoi)ed larva is formed from it, by the overgrowth of this portion 

 of the sclerite and a pushing forward of the process on to the part of the 

 sclerite lying in front of it, aii<l its fusion with the side walls. The skins being 



