136 INJURIOUS AND USEFUL INSECTS 



is ready the husk cracks, and the pupa wriggles out. Leaving 

 the husk behind, it chmbs for some distance up the sheath, 

 its thin and flexible pupa-skin being here, as in the harlequin- 

 fly or the gnat, no impediment to the movements of the 

 body-segments ; then, with the help of certain spines on 

 the prothorax, it tears through the sheath and works its 

 body out until it is half-exposed. The pupa-skin now cracks, 

 and the fly proceeds to extricate itself. Mating and egg- 

 laying follow. The whole life-history may, under favourable 

 circumstances, be transacted in as little as eight weeks; but 

 there is commonly a long pause, and the pupa remains 

 motionless within the flax-seed until the young wheat or 

 barley is out of the ground. The fly is about 3 mm. (^ inch) 

 long ; it has big antennae, every joint of which carries a 

 whorl of hairs, large wings fringed with fine hairsj and long, 

 slender legs. The male fly is smaller than the female, and 

 much more active. 



Since the Hessian-fly scare of 1886, not very much has been 

 seen of this pest in Great Britain. Wheat is not sown in this 

 country till after the date (end of September) when the autumn 

 flies appear. Barley has suffered in particular years, but the 

 attacks have not hitherto approached in severity those which 

 visit the American farmer. Should rigorous measures be 

 called for, the dormant or flax-seed stage offers good oppor- 

 tunities for destruction of the insect. The flax-seeds which 

 drop out during thrashing should be burnt or boiled, the 

 stubble of infected fields fired, and couch-grass thoroughly 

 extirpated. 



The Hessian-fly is the victim of many minute Hymenopterous 

 parasites, wliich are probably one chief means whereby its 

 numbers are kept within bounds. 



28. (PRACTICAL.) THE STRUCTURE OF THE BLOW- 

 FLY {Calliphora etythtocephala or C, vomitoria) 



An ample supply of blow-flies in every stage can be easily 

 procured. Send to a butcher for a piece of fly-blown meat, 

 and keep it out-of-doors under a wire screen until the maggots 

 turn to pupae, and the pupae to flies. Specimens of each may 

 be examined fresh, and others preserved in alcohol for future 



