76 



CAUDAL GILLS OP ZYGOl'TfeRID LARV.E, 



half the thickness of the cuticle, i.e., averaging only 4/x in thick- 

 ness. It is everywhere so deeply pigmented that it is almost 

 impossible to distinguish either cell-boundaries or nuclei. This 

 pigment consists of minute granules of a blackish substance de- 

 posited in all parts of the cytoplasm of the hypodermis. It is 

 only when the hypodermis is cut tangentially, so as to expose 

 the polygonal outline of its cells, that it can be seen definitely 

 that the pigment is less dense along the cell- 

 boundaries, and densest towards the middle 

 of each cell, especially around the nuclei. 

 The latter are of the usual oval shape, and 

 vary from 8 to 10/a in length. 



The distribution of trachea? can be seen 

 from Text-fig. 13. The main trunks pursue a 

 somewhat tortuous course, often coiling round 

 one another. They give off large irregular 

 branches, which run obliquely outwards to- 

 wards the margin of the gill. These large 

 branches again break up very irregularly into 

 numerous smaller branches, which approach 

 the hypodermis at all points of the gill, and 

 finally turn to run parallel with it, giving off 

 numerous capillaries. The courses of these 

 latter cannot be followed owing to the density 

 of the hypoderm pigment-layer. 



The differences between the gills in the 

 three known larvae are only small, but appear 

 to be of sutficient value to separate the species. 

 D. lestdides 8elys, has the longest gills (10-5- 

 11mm.), with the tips more slenderly pointed, 

 and less hairy, than in the other two species. 

 The lateral gills of D. lestdides are also usually 

 distinctly triquetral, the inner faces being definitely flattened 

 against the median gill. D. nymphdides Tillyard, has hairier 

 gills, 9-5-10 mm. long, somewhat less pointed, and generally much 



* Left lateral gill of Diph/ehla hxfdkhx Selys, cleared to show tracheal 

 system; ( x 8). 



'IVxt-tig.i:i 



