DISEASES OF WHEAT CAUSED BY INSECTS. 117 



as " excess of heat or cold, drought or wet, sudden changes 

 of temperature, poverty of soil, over-manuring, shade, &c. 

 tends also to the liability of the crop to be attacked by rust 

 and mildew. 



CHAPTER THIRD. 



DISEASES OF WHEAT CAUSED BY THE ATTACKS OP INSECTS. 



1 4. Peculiarities of Insect Life. Before describing the 

 particular insect pests of the wheat crop, it will be useful, 

 as well as interesting, to glance very briefly at some of 

 the peculiarities of insect life. The science of entomology 

 has only of late years been cultivated with a marked de- 

 gree of success ; previous to this development of interest 

 in it, it was considered by men of science as a department 

 of natural history beneath their notice. It is now, how- 

 ever, recognised as a science bearing with it many impor- 

 tant interests, more especially of those connected with the 

 arts of agriculture and horticulture. An insect, has been de- 

 fined as " an animal with a vertebra, six-footed, with a dis- 

 tinct head furnished by two antennae a pair of confined 

 immovable eyes breathing through openings leading to 

 internal air-tubes or trachece sexes distinct adult state at- 

 tained through a series of changes called "metamorphosis." 

 What these changes are, or of what this metamorphosis 

 consists, we now explain. The first condition is the "egg ; " 

 from this is produced the second change, the "larva" po- 

 pularly known as a caterpillar; from the larva the "pupa," 

 or chrysalis, is produced, and from the pupa the " imago," 

 or perfect insect. In fig. 12 we illustrate the "larva" or 

 caterpillar condition ; in fig. 1 3 the pupa or chrysalis ; in 

 fig. 14 the imago or perfect insect, as the "wheat midge." 

 Of the various orders into which insects are divided we 

 have not space here to enter; nor is it necessary for the 

 purposes of our treatise. Suffice it here to allude to those 



