BY VERA IRWIX-SMITH. 429 



very minute, the exact structure of the parts can only be made out with 

 difficulty, under high magnifleations ; and for a full working out of the details 

 of development, a complete series of the embryonic stages would be required. 

 Unfortunately, I have been able to liatch out only one batch of eggs, and of 

 these, have only the last stage, immediately before hatching, for examination. 

 At hatching, the mouth parts of the larva differ ver>' slightly from those of the 

 older larvae. Some dozen or more skins were obtained, closely tangled uj) with 

 the empty egg mass; aiid all show exactly the same stage of development. 



In the casts, the terminal portion of the pharynx is already surrounded by 

 a thickened outgrowth of chitin, but from this (joint the pharynx runs liaclj 

 through the head as a simple structure of imiform width (Text-fig. 17). None 

 of the skins contain any trace of an internal chitinous skeleton, or indication 

 of the presence of the masticatory apparatus, which is such a prominent and 

 characteristic feature of the larva. 



The Pharynx. (Plate xxxiii., figs. 1-5, and Text-figures 18-26.) 



Vaney, Becker, and Jusbaschjanz have described this apparatus (calletl by 

 them the "Sclilundkopf") as it appears in the aquatic larvae which they 

 examined; and Jusbaschjanz has studied it by means of transverse sections, and 

 has given some figures of it. It is a highly complicated and beautifully adjusted 

 piece of mechanism, and requires a good deal more study than has been given 

 to it. The figures and descriptions already published are not vei-y clear, l)ut 

 in Metoponia n(briceps the apparatus seems to differ somewhat, in form and 

 structure, from those previously described. Satisfactory sections through this 

 part of the larva are difficult to obtain. The chitin composing it is so dense 

 and brittle as to defy all attempts to soften it sufficiently for the microtome, 

 and the parts become shattered in the cutting. Dissections show the relation- 

 ships of the various parts very well, but the study of transverse sections has 

 necessitated much cai-eful reconstruction, after detailed examination of long 

 series of torn fragments. 



As in all Stratiomyid larvae, the dorsal sclerite of the head is continued 

 back through the first thoracic segment as an internal skeleton in the form 

 of a dorsally convex arch, the dorsal head plate {d.p.). It extends to the 

 middle of the second segment, and is firmly held in position by means of power- 

 ful muscles attached to the body wall, which ai'e shown in Text-fig. 20 and Plate 

 xxxiii., fig. 1. Its ventral surface serves as the point of attachment for the dilator 

 muscles of the pharynx (d.m.). 



The pharynx {ph.) lies in the space between its lateral margins, and ter- 

 minates posteriorly about the middle of the first segment, in the complicated 

 masticatory apparatus already referred to. From this, the slender, thin-walled 

 oesophagus passes straight back, below the dorsal plate, to open into the 

 proventriculus at the level of the third thoracic segment (Plate xxxiii., fig. .5). 



In transvei-se sections, stained with Iiaematoxylin and eosin, the dorsal 

 head plate is seen to consist of several layers of chitin (Text-fig. 18). In cutting, 

 it frequently splits apart, along the middle line, into an upper and lower por- 

 tion, of equal width, each pai't graduating from a pinkish colour on the inside 

 to a dark yellowish brown on the outside; and in contact with the latter layer 

 all round, dorsally and ventrally, is a layer of hypodermis. The lateral edges 

 are rounded and thickened, with an additional layer of brown chitin along the 

 upper surface (Text-fig. 19). 



Anteriorly, the walls of the doreal .sclerite close in on the pharTOX, but, so 

 far as I can make out, there is no connection with it such as Jusbaschjanz 



