STRUCTURE, GROWTH AND ECONOMICS OF INSECTS 1 7 



tive value (Fig. 22). In addition, the beautiful colors are due to 

 the scales, and are produced (i) by the pigments present, (2) by the 

 reflection, refraction or diffraction of light, or (3) by a combination 

 of these causes. Scent glands, called aiidroconia, are associated with 

 the scales, mainly on the wings of males. 



(c) Abdomen. — The abdominal segments show a greater uniformity 

 in form than do the thoracic segments, owing to the fact that the former 

 are not speciahzed for the bearing of legs and wings. This uniformity 

 is not merely external but extends to some of the internal organs, 

 notably the muscular, respiratory and nervous systems. A greater 

 differentiation and consequent difference from the other segments is 

 usually found in the terminal segments which are modified to bear the 

 genitalia and cerci, in the first segment in the Hymenoptera which has 

 united closely with the thoracic mass, and in the petiole of the same 

 insects which is greatly reduced in diameter and often nodulated (ants). 



Typically the abdomen consists of ten segments, but in some 

 Orthoptera eleven have been found and twelve in a few embryos. In the 

 adult insect it is often difficult to distinguish ten abdominal segments 

 owing to the fusion or disappearance of certain of the segments, to the 

 modification which the terminal segments undergo, or to the telescoping 

 of these last within the other segments. Each segment is made up of a 

 dorsal Sclerite, the tergum, a ventral sclerite, the sternum, and a pair of 

 pleural membranes connecting the two. The first seven or eight 

 abdominal segments usually bear a pair of spiracles each. Typically 

 the anus opens in the tenth or last segment and the reproductive system 

 between the eighth and ninth. 



Appendages of the Abdomen. — In the embryo the abdominal seg- 

 ments bear paired appendages which are homodynamous with the legs 

 and mouth-parts. These usually for the most part disappear on hatch- 

 ing, but they persist as prolegs during the larval Ufe of the Lepid- 

 optera, Tenthredinidae and Mecoptera; in certain Thysanurans they 

 persist as rudimentary abdominal appendages throughout the life of 

 the insect. 



The cerci which are present in most of the more generalized orders 

 and in the thysanuriform larvae are usually the appendages of the tenth 

 abdominal segment. 



The gonapophyses or plates of the genitalia are the appendages of 

 the seventh, eighth and ninth segments and are known respectively as 



