70 



Bentinck: Structure and Classification 



Fig. 1;2« Mouth parts of Pteronarcys cal ifornica 

 Newport (Essig, 1942). 



ach. The hind-gut is differentiated into the small 

 intestine with the Malpighian tubules, or excretory 

 organs, arising from its anterior extremity, and the 

 rectum terminated by the anus. 



The heart is an open tube lying dorsally above the 

 alimentary canal. 



The paired salivary glands lie anteriorly beneath 



Rect 



An 



Va 5 



Fhy / \ X SalGl 



'imMth Hphy 



VNC 



SInt 



the intestine and open via a common duct on the 

 hypopharynx. 



The reproductive system consists of a pair of 

 gonads usually connected by a common duct leading 

 to the exterior. Accessory structures in the female 

 include one or more spermathecae or sacs for storage 

 of seminal fluid, and colleterial glands for secretion 

 of cement, gelatin, or other substance for attaching 

 or protecting the eggs. The external structures con- 

 sist of appendages of the caudal segments variously 

 modified as an ovipositor in the female or as a clasp- 

 ing organ in the male. 



The central nervous system consists of a paired 

 ventral nerve cord connecting segmentally arranged 

 ganglia. The first ganglionic mass which constitutes 

 the brain lies in the head above the esophagus. It 

 is connected with the second mass or subesophageal 

 ganglion by a pair of connectives. In the more primi- 

 tive insects the postcephalic ganglia tend to be 

 segmentally arranged, but in the more specialized 

 groups there is a tendency for consolidation of these 

 ganglia into a single mass in the thorax. 



Respiration is accomplished in most insects by 

 means of the tracheal system, a network of fine 

 tubes by means of which air is brought directly to 

 the tissue cells. Typically the tracheal system con- 

 sists of two longitudinal tracheal trunks running the 

 length of the body, one on each side. Smaller branches 

 lead to the external openings or spiracles, two pairs 

 of which are found in the thorax and up to eight pairs 

 in the abdomen. The lateral trunks give rise to numer- 

 ous branches which divide many times and penetrate 

 the tissues. This system may be developed to various 

 degrees; it may function in conjunction with gills in 

 aquatic forms, or it may be entirely lacking. In the 

 latter case respiration may proceed by diffusion 

 through the cuticle. 



The fat body or adipose tissue occurs throughout 

 the body and often completely surrounds the alimentary 

 canal and other organs. Its primary function appears 

 to be storage of nutrients. 



GROWTH AND DEVELOPMENT 



dorsal tracheal trunk 

 dorsal branch 



ventral 

 diaphragm 



dorsal commissure 

 alimentary canal 



' dlaptvogn 



salivary 



ventral com™ 



trvnk 



aleral branch 

 vanl ml tracheal trunk 

 oanojttn 



Fig. 1:3. Diagramatic sections through an insect body showing 

 arrangement of principal internal organs, a, sagittal section; b, 

 cross section through thorax. An, anus; Br, brain; Cr, crop; 

 Hphy, hypopharynx; Ht, heart; Lb, labium; Lm, labrum; Mai, 

 Malpighian tubules; Mth, mouth; Oe, esophagus; Pvent, proventri- 

 culus; Rect, rectum; SalGl, salivory gland; SInt, small intestine; 

 Soe Gng, subesophageal ganglion; Vag; vagina; Vent, ventriculus 

 or stomach; VNC, ventral nerve cord (a, Snodgrass, 1925; b, 

 Essig, -1942). 



From the time of hatching until adulthood the individ- 

 ual passes through a period of growth and change. 

 The insect integument has little capacity to stretch, 

 so to accommodate increase in size the insect periodi- 

 cally sheds or molts its skin and replaces it with a 

 larger one. The process of molting is sometimes 

 called ecdysis, and the castroff skins are called 

 exuviae. Most insects molt three to six times during 

 normal development but in some cases thirty or more 

 molts occur. The period between any two consecutive 

 molts is called a stadium or instar. The period between 

 hatching and the first ecdysis is the first stadium, 

 and any individual in this stage of development is 

 a first instar. 



The adult or imago is the stage having functional 

 reproductive organs and associated mating or egg- 

 laying structures. It is the stage that usually bears 

 fully developed or functional wings. 



