INSECTA 391 



are the various shaped feelers (antennae) which are placed on the superior 

 surface of the head next to the eyes ; then more ventrally placed a pair 

 of upper jaws (mandibles) without palpi and without articulations ; they 

 are powerful masticatory organs. 1 The first pair of lower jaws (maxillae) 

 are jointed and bear a palpus (palpus maxillaris) ; the second pair of 

 maxillae are soldered together and form the lower lip (labium), and likewise 

 carry a palpus labialis on each side. The upper lip (labrum") as well as 

 a few other pieces, which, however, are only appendages, belong to the 

 mouth, which is really formed of a number of closely-united pieces. 

 The mouth parts undergo manifold metamorphoses according to the func- 

 tions required of them. Coleoptera, Neuroptera and Orthoptera have 

 biting or masticatory mouth parts which conform with the scheme described 

 above. In the licking mouth parts of the hymenoptera the maxillae and 

 under Up are considerably elongated, while the mandibles retain their form 

 and are used for triturating the food : in the Lepidoptera nearly all the 

 mouth parts are shortened except the maxillae, which form a long and some- 

 times spirally rolled suctorial proboscis ; the Diptera and Rhynchota have 

 piercing and sucking mouth parts. The mandibles and maxillae are meta- 

 morphosed into a needle-like organ, while the suctorial apparatus is formed 

 by the labrum (compare p. 97). 



The thorax consists of three segments which are frequently completely 

 united ; ventrally it carries three pairs of legs, which consist of a definite 

 number of articulated pieces joined one to the other. Their form also 

 changes according to their function, so that legs for running, walking, 

 digging, swimming, jumping, and preying are seen. A pair of wings are 

 respectively joined to the last and last but one thoracic rings, and these 

 may be traced back, not to metamorphosed appendages, but to tracheal 

 branchia. They are composed of chitinous membranes supported by branched 

 structures (veins or ribs). Their size and formation vary ; they are seldom of 

 equal size and form (Neuroptera) ; often the posterior wings are larger than 

 the anterior wings, the former then' only serving as protective coverings for 

 the latter (Coleoptera), or the anterior wings are larger (Lepidoptera), or the 

 posterior wings are shortened, or are entirely absent (diptera) ; and finally 

 there are insects in which both pairs of wings are lacking. 2 



The abdomen retains its segmentation, but, with the exception of a 

 few groups related to the primitive forms of insects, has no appendages in 

 the imago condition ; the abdomen usually consists of ten segments, on 

 the last of which the anus is situated. 



We need only observe the following points in considering the anatomy 

 of insects. 



The EPIDERMIS consists of the chitinous cuticle, which is separate from 

 the cellular layer beneath (hypodermis) ; the various appendages are sup- 

 ported by the chitinous layer. 



1 [The mandibles are only powerful masticatory organs in biting-mouthed insects 

 (Mandibulata); in the sucking or piercing-mouthed insects they may be absent or 

 in the form of needle-like styles (Haustellata). F. V^ T.] 



- [As in the order Aptera, which includes the Thysanura and Collembola, and also 

 exceptions in other orders, as the fleas amongst Diptera ; the Mutillas and ants 

 amongst Hymenoptera. F. V. T.] 



