498 THE ANIMAL KINGDOM. 



somites are firmly united. The pro-thorax is generally small, the meso- 

 and meta-thorax either equal in size or one larger than the other, the 

 variations depending on the peculiarities, &c. of the wings. The thoracic 

 somites, especially the meso- and meta-thorax, are rarely ring-like (Aptery- 

 gogened), but have well-developed tergal and pleural regions. The sternal 

 region varies much in size. The abdomen consists typically of eleven 

 somites, but the number may be reduced in certain groups, e.g. to ten 

 in Lepidoptera &c., to nine or eight in many Diptera. The successive 

 somites are connected by soft intersegmental membranes, which admit 

 of contraction and expansion of the abdomen in respiration and of its 

 distension by the sexual products. Each somite of the abdomen may 

 have the form of a ring (Apterygogenea ; Plecoptera), or may have a firm 

 tergal and sternal plate connected laterally by soft pleural membranes. 

 The shape of the abdomen varies much. The first somite may remain 

 independent, e. g. Dermaptera, Ephemeridae, or it may lose its sternal plate 

 whilst the tergal plate becomes connected to the meta-thorax, e. g. 

 Coleoptera, Trichoptera, a change that may extend to more or fewer of 

 the following somites. In Hymenoptera with a pedunculate abdomen 

 the peduncle is formed by the contracted second, or second and third 

 somites. The meta-thorax in Macro-Lepidoptera, Diptera, and Rhynchota 

 Homoptera is so closely united to the first abdominal somite that in 

 most instances it appears to be a portion of it, the meso-thorax being very 

 large. 



The head bears the antennae, mandibles, maxillae, labium or second 

 maxillae. The antennae, processes as in Myriapoda of the procephalic 

 lobes and not appendages, are borne on the margin of the head in front 

 of the eyes in more primitive forms, or upon the vertex, i. e. summit of 

 the head. They are jointed, and their length, shape, and other secondary 

 characters are very variable. They lodge peculiar nerve-endings and appear 

 to be olfactory in function. The mouth-parts may be similar in all stages 

 of life and then either adapted for biting (Menognatha) or for sucking 

 (Menorhyncha), or else in the larva they are adapted for biting, in the 

 imago for sucking (Metagnathd), the change commencing in the pupal, 

 and rarely affecting the larval, stage. The mandibles are one-jointed and 

 never possess a palp, and are furnished in biting insects with a variously 

 conformed biting or masticatory surface, whilst in sucking insects they 

 may be reduced to a pair of stilets (i. e. are setaceous), e. g. Rhynchota, 

 become rudimentary as in many Diptera, or even completely lost as in 

 Lepidoptera with rare exceptions (see p. 150). The maxillae have a com- 

 plicated structure when fully developed as in biting insects (see p. 140), and 

 are provided with jointed palpi 1 . The two galeae are lengthened out, and 



1 Cf. Beauregard on Vesicant Insects, Journal de 1'Anat. and Physiol. xxii. 1886, p. 95 et seqq. 



