Reduction of Larval Head in other Neuiocera 31 



larval epidermis,, in a way wliicli will be particularly 

 described hereafter, and mucli more space is required for 

 these folds than the small, hard head of the Chironomus- 

 larva can supply. Since the imaginal head has to enclose 

 the brain, it must form about the larval brain, and this 

 makes it intelligible that in certain species of Chiro- 

 nomus the larval brain and the rudiments of the 

 imaginal head should both shift into the relatively 

 spacious prothorax. It may well be that the removal of 

 these parts has led to a further reduction of the larval 

 head. 



Many Nemoceran larvae, including some Chironomus- Eeduction 

 larvae, have a well-developed head, which lodges the brain j'^g^^^^u'^ 

 and sub-oesophageal ganglion, bears eyes or eye-spots, an- other 

 temiae, and three pairs of jaws, and is externally defended Nemocent. 

 by a dense and complete chitinous armour. The eyes are 

 often "compound in the larvae of Culicidae (Culex, Ano- 

 pheles, Corethra, Mochlonyx). But where the larva is 

 addicted to burrowing, and especially where it buries 

 itself in its food^ the head undergoes more or less reduc- 

 tion in size, which is nearly always associated with 

 complete or partial retraction into the thorax. Some- 

 times only the hinder part of the head is retractile, and 

 then its chitinous cuticle becomes thinner, or is excavated 

 by notches, as if only those parts which serve for 

 muscular attachment were retained. Larval head-reduc- 

 tion is not unknown in Nemocera, but it is universal, so ^ 

 far as we know, in Brachycera, where it is often carried 

 much further than in any Nemocera. The back j)art oi 

 the retractile head shows, at least when not extremely 

 reduced, a median and a pair of lateral projections, the 

 remnants of a continuous cephalic shield. Any of the 

 three principal divisions may be again subdivided. In 

 heads which are still further reduced the principal parts 

 which remain are not threefold, but paired, and are, we 

 are inclined to think, rather paired apodemes than 

 remnants of the cephalic shield. There are often two 

 such pairs, which are long, slender, and exclusively con- 

 cerned with muscular insertion. In extreme cases, e.g. 

 in the leaf- mining larva of Phytomyza, only a single pair 

 remains, and this is reduced almost beyond recognition. 



