918 



INSECTA. 



Fig. 388. 



Internal skeleton of Lucamis census. 



meron, it is formed by a reflection inwards of 

 part of the anterior margin of the meta-sternum, 

 (4 q,} with which the meso-sternum has become 

 anchyclosed, and the episternum and epimeron 

 form the lateral boundary of the cavity. 

 The great strength of limb required by insects 

 for other purposes than those of locomotion, be- 

 longs especially to the first and second pair of 

 legs, and consequently the articulations of 

 these with the body are required to be most 

 secure. We have seen that in the aquatic in- 

 the posterior pair are almost solely em- 

 ployed m swimming, and in the terrestrial in- 

 sects they are in like manner employed chiefly 



in locomotion. The necessity, therefore, fot d 

 consolidation of the walls of the acetabula, 

 into which they are inserted, is not so great as 

 in the preceding instances, and consequently we 

 find that those for the posterior pair (4 r) are 

 formed by the posterior margin of the expanded 

 meta-sternum in front, and the consolidated 

 margin of the inferior surface of the fifth or 

 thoracico-abdominal segment behind, reflected 

 inwards and upwards, and loosely articulated 

 in the median line with the sternum, thus al- 

 lowing of the freest motion to the coxae, the 

 sides of each being formed by the epimeron. 

 But in insects which move with a sudden 

 effort, as in jumping, and in those that employ 

 the hinder legs as prehensile organs, like the 

 Coprida, Ateucln, and others, these legs, like 

 the anterior ones, are inserted into deep 

 acetabula. 



The abdomen, or third division of the body, 

 is entirely destitute of organs of locomotion. 

 It contains the chief part of the digestive, re- 

 spiratory, circulatory, and generative systems, 

 and, like the thorax, is composed of distinct 

 segments. These are nine in number, if the 

 fifth segment of the body, which almost disap- 

 pears during the change to the perfect state, be 

 included. This segment, however, we prefer 

 to consider as a distinct part, so that the abdo- 

 men consists certainly of eight segments, be- 

 sides the anal appendages. Each segment is 

 formed of one dorsal and one ventral plate, 

 connected at the sides by a distinct membrane. 

 Only five of these plates are in general to be 

 observed on the under surface, but some trace 

 of the whole number is always seen on the 

 upper, and also at the sides (jig. 388). This 

 arises from the circumstance that a portion of 

 the ventral surface of the first three segments of 

 the larva is employed in forming the under 

 surface of the anterior part of the abdomen of 

 the perfect insect, the change in Coleoptera, as 

 in other insects, being carried to a greater ex- 

 tent on the under than on the upper surface of 

 the body. One segment, also, the anal one, 

 becomes partly removed from the others at the 

 posterior part, and is retractile within the ab- 

 domen, so that there are only five connected 

 segments to the ventral surface, but nine on the 

 upper. The form of the abdomen is in general 

 somewhat triangular or oval in Coleoptera, its 

 basial part being of the same width as the tho- 

 rax. Each segment is freely moveable, the an- 

 terior part of one being retractile within the 

 posterior of another. At the external margin 

 of each dorsal plate, in the membrane that 

 connects those of the upper with the under 

 surface, there is an oval corneous ring, the 

 xpiraculvm^ or breathing orifice, which commu- 

 nicates internally with the organs of respiration. 

 In most of the Coleoptera the abdomen is co- 

 vered by the elytra, but in some species it is 

 exposed, as in the oil-beetles, rove-beetles, and 

 ear-wigs. In the latter instances it is furnished 

 at its extremity with strong forceps, which 

 appear to be analogous to parts which we are 

 about to consider more particularly in other 

 insects. 



