12 



FOREST ENTOMOLOGY. 



femur ; then we have a long joint called the tibia ; and finally, the 

 joints (five or fewer) which make up the tarsus ov foot. Fig. 13 is a 

 representation of the different parts of the leg. 



The foregoing characters represent the more important points as 

 regards external structure, but it is somewhat more difficult to deal 

 with internal structure and function. To deal with too many details 

 would simply embarrass the ordinary student, and at the same time 

 the subject cannot possibly be intelligible to the student if too few 

 points are presented. 



The accompanying diagram (fig. 14), representing a longitudinal 



Fig. 14. — Diagram of longitudinal section of an insect, shoiring principal anatomical points of an 

 insect. (From ' Text-Book of Zoology,' by Dr J. B. V. Boas.) 

 1-3, first and third pairs of legs cut away; o, anus; c, cerebral ganglion; ch, mesenteron ; 

 e, proctodseum ; g, genital aperture; h, heart; k, crop; m, mouth; «, ventral ganglion; 

 sp, salivary gland; «, uialpighian tubule; a', ovary. 



section of an insect, shows the relative position of the more important 

 anatomical points from a side view. 



The nervous system of an insect resembles that of most other 

 animals, inasmuch as the terminal seat of the nerve-centres is the 

 brain, whence proceed other nerve-centres and nerves all over the 

 body. The nerves receive impres- 

 sions from the outside world, which 

 react on the organism, and thus stim- 

 ulate or restrain muscular action. 

 Fig. 15 shows a photograph of a 

 horizontal section through the head 

 of a blow-fly. 



The various convolutions of the 

 brain should be noted ; and as the 

 development of these convolutions is indicative of intelligence, it is 

 no wonder that certain insects, in this respect, are considered as 

 approaching nearer to man than many other animals. 



The respiratory organs of an insect are composed of a system of 



Fig. 15. — Horizontal section through the head 

 of a Mow-fly, showing eyes and brain. 

 (Section and photo by A. Flatters.) 



