I 



INSECT A. 501 



in Dipterous larvae are regarded by Brauer as secondary structures and not 

 as homologues of true appendages. 



The chitinoid cuticle is generally firm, but its texture varies in different 

 orders and in different stages of life. Copper has been detected in it 

 in Coleoptera. Calcareous hardening is rare. It is more or less marked 

 by hexagonal areae and by various sculpturings (dots, pits, lines, &c.). 

 It is frequently covered with hairs, and scales placed above fine pores. 

 Tactile hairs of different shapes, connected basally with a cell and a nervCj 

 are found on the integument of various soft-bodied and aquatic larvae and 

 in certain parts of the imago, e. g. on the antennae, palpi of maxillae and 

 labium, wings, halteres, tarsi. Thin spots of the cuticle with underlying 

 cells in connection with nerves are found also on the palpi. Integumental 

 glands are not common, but are found in various regions of the body, 

 e. g. scent-producing glands sometimes connected with hairs ; wax-pro- 

 ducing glands distributed over the body, e. g. in certain Aphidae and 

 Coccidae (Homopterd], or confined to the ventral surface of the abdomen 

 in Apis ; poison glands in connection with the sting in Aculeate Hymen- 

 optera (Bees, Wasps, Ants), or with hairs as in some Lepidopterous Cater- 

 pillars, &c. 



The supra-oesophageal ganglion supplies the antennae and eyes. It 

 has a complicated structure in the imago, especially in the higher orders 

 e. g. Hymenoptera. The infra-oesophageal ganglion innervates the oral 

 appendages, glands of the mouth, &c., and represents at least three ganglia 

 fused. The ventral chain consists of a series of ganglia. The last probably 

 always represents two even in primitive forms with the ganglia separate, 

 a condition most generally found in the cruciform larva. Fusion may 

 take place berween the meso- and meta-thoracic ganglia, between some 

 or all the abdominal ganglia which may then become placed in the thorax 

 (Heteroptera, some Dipterd) ; but it is rare for the infra-oesophageal ganglion 

 to unite with the fused thoracic and abdominal ganglia, as in the parasitic 

 Pupipara among Diptera. The nervous system of the larva may be more 

 or less concentrated and little differentiated, and may become less con- 

 centrated and more differentiated in the imago, e. g. Miisca, Myrmeleo. 

 As a rule however the imago possesses in the Metabola a more concentrated 

 and specialised nervous system than the larva. There is a stomatogastric 

 sympathetic system divisible into an azygos portion and a paired portion, 

 conformed much as in the Cockroach (see p. 142-3) and derived from the 

 supra-oesophageal ganglion. A respiratory sympathetic system regulating 

 the closure of the stigmata, &c. originates from a nerve in relation with 

 the dorsal aspect of the ventral chain (p. 149). 



Few Insecta are blind, e. g. some subterranean and cave insects, &c. and 

 some larvae. Eyes are restricted to the head and occur in two forms, 

 the monomeniscous ocellus and the polymeniscous or so-called compound 



