SEGMENTS 



there is one of these spiracles. This, however, is a rule subject 

 to many exceptions, and it is doubtful whether there is ever a 

 spiracle on the last abdominal segment. Even in the young stage 

 of the Insect the number of these stigmata is variable ; while 

 in the perfect Insect the positions of some of the stigmata may be 

 much modified correlatively with the unequal development or 

 consolidation of parts, especially of the thorax when it is highly 

 modified for bearing the wings. 



The segments of the Insect are not separate parts connected 

 with one another by joints and ligaments ; the condition of the 

 Insect crust is in fact that of a continuous long sac, in which 

 there are slight constrictions giving rise to the segments, the 

 interior of the sac being always traversed from end to end by a 

 tube, or rather by the invaginated ends of the sac itself which 

 connect with an included second sac, the stomach. The more 

 prominent or exposed parts of the external sac are more or less 

 hard, while the constricted parts remain delicate, and thus the 

 continuous bag comes to consist of a series of more or less hard 

 rings connected by more delicate membranes. This condition is 



FIG. 48 Tillus elongatus, fully distended larva. 



readily seen in distended larvae, and is shown by our figure 48 

 which is taken from the same specimen, whose portrait, drawn 

 during life, will be given when we come to the Coleoptera, family 

 Cleridae. The nature of the concealed connexions between the 

 apparently separate segments of Insects is shown at m, Fig. 47, 

 p. 88. 



As the number of segments in the adult Insect corresponds 

 except in the head with the number of divisions that appear 

 very early in the embryo, we conclude that the segmentation of 

 the adult is, even in Insects which change their form very greatly 

 during growth, due to the condition that existed in the embryo ; 

 but it must not be forgotten that important secondary changes 

 occur in the somites during the growth and development of 

 the individual. Hence in some cases there appear to be more 

 than the usual number of segments, e.g. Cardiophorus larva, 

 and in others the number of somites is diminished by amal- 



