DISEASES OF CROPS. 



The majority of the animal foes which cause disease in 

 crops belong to the great class of Insecta ; a few are mem- 

 bers of the Vermes (worms) and the Myriapoda (milli- 

 pedes). 



" Insects in their most complete character pass through 

 four stages or phases of existence — the egg, the larva 

 (maggot or caterpillar) stage, the chrysalis, and the 

 perfect state. In none of these, except the larval or 

 caterpillar stage, does the insect increase in size. . . . 

 After insects have come out of the chrysalis stage they 

 never grow — all the growth is done in the earlier stage 

 when they are caterpillars. If we sometimes meet with 

 two insects of the same species but of different size, the 

 difference is due to the supply of food which the cater- 

 pillar had during its growth, and is only a parallel case 

 to an ill-nourished child growing up into a stunted man. 

 Some insects, as the Aptera (wingless insects), pass only 

 through three stages : the egg, the ' younger stage ' and the 

 perfect form ; and some of the intermediate orders also 

 attain perfection without passing through more than two. 



" The egg is usually deposited externally, but in some 

 few cases it is hatched in the body of the parent ; in some 

 others it is deposited at one period of the year, and the 

 progeny brought forth alive at another period. . . . 

 In the larval stage, the insect casts its skin or moults 

 several times, after each casting attaining a sudden and 

 rapid increase of size. The larva does not always take 

 the form of a caterpillar or maggot. In some orders 

 (the Aptera, Hemiptera [bugs] and Orthoptera [cockroach, 

 dragon-fly, thrips]) it assumes a good deal of the appear- 

 ance of the perfect insect. In this imperfect metamor- 

 phosis it changes its skin as the caterpillars do, and it does 

 not assume a different form for the chrysalis stage. 



