STRUCTURE AND CLASSIFICATION. 



39 



that dry insecticides penetrate them readily, if finely ground ; 

 and all such are effective in proportion to their fine mechanical 

 condition, enabling them to enter the tracheae through the sieve 

 of the spiracles. Thus, sawfly larvae or " slugs," like the " cur- 

 rant-worm" and " pear-slug," succumb readily to dry hellebore, 

 pyrethrum, or even fine road dust. Many parasites may be 

 reached in this way, and some animals and all fowls keep them- 

 selves tolerably free from vermin by frequent dustings. This is 

 not the place to go into the effectiveness of insecticides ; but it 

 may be repeated that contact poisons usually act through the 

 spiracles only. Dry powders are effective in proportion to their 

 fineness and to their peculiar poisonous properties ; liquids are 

 effective in proportion to the thoroughness with which they are 

 applied, and to their penetrating or clogging characters. 



Caustics and chlorides act differently, and may actually corrode 

 a thin insect crust. But this matter also comes more appro- 

 priately under another heading, and will not be further discussed 

 here. 



CHAPTER VI. 



NERVOUS SYSTEM AND SENSES OF INSECTS. 



The nervous system in insects consists of a series of white 

 disks or gajiglia lying on the bottom of the body cavity, con- 

 nected by a double cord extending the full length of the insect. 

 Insects have no true brain, as do the higher animals, and the 

 ganglion situated in the head, which is usually called " brain," is 

 larger than the others simply because of the great number 

 of special organs — eyes, antennae, and mouth parts — that must 

 be innervated from it. In its structure it is precisely like the 

 other ganglia, though in some of the social types there is an 

 appearance of specialization. Nerve-fibres start from all ganglia 

 in every direction, and all parts of the body are reached ; the 

 nervous system in one direction being exceedingly well devel- 

 oped. In the larval forms and in the lower types, a ganglion is 

 present in every segment of the body, including the head ; and 

 thus each segment contains its own nerve-centre, the cords from 



