28 



AN ECONOMIC ENTOMOLOGY. 



we meet with a whitish or slightly greenish liquid, much more 

 abundant in the larva than in the adult ; and this is the blood. 

 These various systems are of economic importance, for upon 

 our full understanding of some of them depends the success with 

 which we can apply insecticides of a certain character. 



Perhaps the least important in this view is the muscular sys- 

 tem. It has been stated that the muscles are found in the form 

 of bands, which extend from the body walls to the various ap- 

 pendages, and, of course, they are numerous and strong in pro- 

 portion to the power required. Thus, for a leaping insect the 

 muscles moving the legs are very much more developed than 

 they are in an insect which simply walks or runs, and in a run- 

 ning insect they are better developed than in one that simply 

 walks or moves slowly. Where powerful appendages must be 

 moved, as for instance the mandibles or jaws, several bundles 

 of muscles frequently converge to a chitinous point or tendon, 

 which in turn is attached to the appendage, and thus a great 

 amount of force is exerted at one point, the muscular attach- 

 ments on the body wall covering a considerable surface and con- 

 verging all their effectiveness upon one lever only. It is this 

 arrangement that gives some insects the gnawing power to bur- 

 row in the hardest woods, and to cut through foreign substances, 

 like lead, in order to escape from captivity. 



Under the microscope it is found that the ultimate structure of 

 insect muscle does not differ in essentials from that of the higher 

 animals ; that is to say, it is made up of narrow fibres divided 

 into cells, and transversely striated. The voluntary muscles of 

 insects are, therefore, practically like the voluntary muscles of 

 the higher animals or of men. 



The digestive system is of much more importance from the 

 economic stand-point. On a previous page it has been shown 

 that two general types of feeding habits exist, the chewing and 

 the sucking, and the digestive system changes somewhat as the 

 needs of the organism vary. The mandibulate type, in which 

 the insect chews its food and subsists upon more or less sohd 

 material, will be first considered. Most insects in the larval stage, 

 and sometimes also as adults, are voracious feeders, seeming 

 determined to devour as much as they can possibly contain, in 

 the shortest possible time ; and the food is in such cases rather 



