TYPES OF LIFE -CYCLE 



331 



the outset, the eggs or larvae as the case may be being inserted 

 within the bodies of the hosts. 



Each of these types of hfe-cycle and the adaptive features 

 involved will be briefly discussed. 



Group A. Since the eggs, or, in other cases, larvae, are not 

 actually deposited on the hosts it is obvious that special provision 

 is necessary whereby the parasites are enabled to obtain access 

 to the latter. Such provision is forthcoming either (1) by very 

 high fecundity and the deposition of minute eggs adapted to be 

 ingested by the host during the ordinary process of feeding ; or 



(2) by the primary larval stage being essentially a migratory phase 

 enabling the parasite to discover its hosts by its own efforts ; or 



(3) by the primary larva developing an erect stationary habit and 

 awaiting an opportunity of 



attaching itself to the first 

 suitable host coming within 

 reach. 



1. The Ingestion of Parasite 

 Eggs by the Host. This habit 

 is peculiar to certain of the 

 Tachinidge and its prevalence 

 was made known by the earlier 

 researches of Townsend (1908) 

 and especially of Pantel ( 1 909 ) ; 

 more recently Thompson (1924) has considerably extended our 

 knowledge of the subject. These observers have shown that 

 species which lay very large numbers of minute more or less 

 fusiform eggs (microtype eggs of Pantel) deposit them upon the 

 food-plant of the host. The larvae within these eggs are fully 

 formed, and only await a favourable opportunity for emergence. 

 The species Gonia capitata, a para^site of the prairie cutworm 

 (Porosagrotis orthogonia) in Canada, has been studied by Strickland 

 (1923) and may be taken as an example. It lays upwards of 

 4,000 small black eggs, measuring 0-24 mm. long X 0-15 mm. 

 wide and 0-12 mm. high, which do not normally hatch until 

 they are swallowed by the cutworm larva along with its food 

 (Fig. 75). Upon coming in contact with the digestive juices the 



Fig. 75. Eggs of a Tachinid fly (Gonia 

 capitata) laid on blade of wheat. 

 X 50. a, Adhesive material. 



(After Strickland, Tech. Bull. 26, 

 Dept. Agric.y Canada,) 



