24 



Report on the Diseases of Wheat. 



of the dead caterpillar^ or come out of it, before they assume 

 the fly-state. 



Each species of Ichneumon is restricted in its attacks to 

 one^ or at most to a few, particular species of caterpillar ; and 

 the females instinctively proportion the number of eggs they 

 deposit in each individual to the relative size of their own 

 offspring, and that of the insect on which it is destined to 

 prey. In some cases, a single egg is laid in a caterpillar ; in 

 others, some dozens. These ichneumons are obviously destined 

 to restrain within due limits certain species of insects, which 

 would otherwise inconveniently increase. Our wheat-midge has 

 certainly one, and probably not less than three distinct species of 

 these ichneumons appointed to keep it in check. One of these 

 ( Platygaster tipulce) may be seen in July and August, actively 

 engaged in examining the ears of wheat infested by the cater- 

 pillars of the wheat-midge. It is a minute black four- winged 

 fly, with a sharp point at the tail, which it insinuates between the 

 chaff- scales, and then pierces the caterpillars, depositing a single 

 egg in every one it is able to reach. One ichneumon, therefore, 

 destroys many caterpillars ; and it is clear, if it were not for 

 their friendly interference, our wheat crops would in a few years 

 run a fair chance of being utterly destroyed. If my conjecture 

 is correct, that it is only those caterpillars of the wheat-midge 

 which have been punctured by the ichneumons, which quit the 

 ears and bury themselves in the earth, there is thus a very 

 effectual provision made for the preservation of the ichneumons ; 

 whilst, on the other hand, many of the midge chrysalides must 

 necessarily be destroyed during the process of threshing the corn, 

 and more might easily be got rid of with proper precaution. 

 My conjecture is founded on the following observations made this 

 autumn. I had observed no ichneumons about any of the ears 

 of some wheat which had been attacked by the wheat-midge, and 

 in the specimens which I brought home, many if not all of the 

 caterpillars have spun up in the chaff-scales. But, in some other 

 specimens, obtained later in the season, and on which I had seen 

 the ichneumons busily employed, I found that many of the cater- 

 pillars came out of the ears, and buried themselves in some sand 

 placed at the bottom of the vessel in which I had put them. 

 At present they have neither spun nor changed to chrysalides. 

 In the second volume of the " Magazine of Natural History," 

 p. 292, is a notice by Mr. Gorrie on the effects produced by the 

 wheat-midge ; and he estimates the loss in the late-sown wheats, 

 in Perthshire in 1828, to have amounted to one-third of the crop ! 

 and again, at p. 324, he says they destroy from three to five bolls 

 per acre. He asserts that all the maggots had descended into 

 the earth by the 1st of August, and advises their destruction in 



