32 MANUAL OF VEGETABLE-GARDEN INSECTS 



which the middle lower pair are two-toothed. At the side of 

 the body just back of the head is located a pair of spiracles 

 which appear as brownish fan-like projections each having 

 twelve divisions. These characters are used in separating the 

 cabbage maggot from its near relatives (Fig. 28). 



When full-grown, the maggots work their w^ay into the soil 

 an inch or so, and there the skin contracts, hardens and turns 

 brownish, thus forming the puparium (Fig. 29). Sometimes the 

 maggots make this change in cavities in the roots. Within the 

 puparium there takes place a remarkable series of changes 

 whereby the tissues of the larva are broken down and rebuilt 

 into the organs of the fly. The fly escapes from the puparium 

 through a circular seam at one end; 

 the length of the period passed in the 

 puparium varies greatly; most of the 

 flies emerge in twelve to eighteen days ; 

 Fig. 29. — Puparium of a few may appear sooner, and a con- 

 got ix7)^^ root-mag- gi.ierable number emerge irregularly 

 throughout the remainder of the season. 

 A few puparia of this first brood may hold over till the 

 following spring. A second brood of flies appears the last 

 of June and throughout July. A third brood emerges from 

 August till October. In some seasons a partial fourth brood 

 may occur. As a rule it is the first brood of maggots that 

 is most destructive to cabbage, cauliflower, radish and 

 turnip, although occasionally the second brood causes seri- 

 ous damage. The wild plants in which breeding takes place 

 are hedge mustard, white mustard and winter cress or yellow 

 rocket, and probably other members of the mustard family, 

 although shepherd's purse seems to be immune. In Canada the 

 flies have been reared from maggots infesting the roots of bean 

 and beet. In the North, as a rule, the insect hibernates in the 

 puparium stage, but occasionally a few of the flies emerging late 

 in the season may pass the winter under the protection of the 



