178 Life- History of the Onion Fly 



Life-History. 



The Egg. Description. The egg of the Onion-Fly is white in colour 

 and 1 mm. in length. The outer coating is ridged and there is a shallow 

 depression down one side extending about a third of the distance. The 

 egg much resembles that of Ckortophila brassicae, the Cabbage Root Fly, 

 except that it is larger and the depression is shorter and shallower than 

 is that in the egg of the Cabbage-Fly. PI. X, fig. 1 shows the egg of the 

 Onion-Fly (A) compared with the egg of the Cabbage Root Fly (B). 



Duration of Egg Stage. This period varies according to the tempera- 

 ture. The usual time is about three days but is occasionally prolonged 

 to six or seven days. 



The Larva. Description. On hatching from the egg, the young maggot 

 makes its way through the soil and attacks the root of the onion, boring 

 its way in through the base of the bulb. The full-grown larva and the 

 newly hatched larva do not differ materially except in size. 



When full grown the maggot is from 9-10 mm. long and 1^ mm. 

 broad at the thickest part. It is white in colour, flattened at one end 

 and tapering to a point at the other. At the broad flattened end which 

 is the "tail," are numbers of tubercles arranged as shown in fig. 2. In 

 the centre of the flattened end are two chitinous projections, these are 

 the posterior spiracles or "breathing pores." Anteriorly, at the "head" 

 end, is a pair of black hook-like "jaws" of strong chitin by means of 

 which the larva bores its way into the onion. These "jaws" are con- 

 tinuous with a chitinous framework to which are attached a number of 

 muscles; surrounding the hooks on the outside of the "head" is a pair 

 of large fleshy lip-like structures. There is also a pair of small 

 papillae. A little further back are the anterior spiracles, these consist 

 of two flattened fan-like outgrowths, one on each side. Each spiracle 

 is composed of eleven finger-like lobes, this number is not constant but 

 varies in different larvae. Fig. 3 is a drawing of the anterior end of the 

 adult maggot and fig. 4 is an enlarged photograph of the whole insect. 



Length of Larval Period. From a number of observations made on 

 the length of the larval stage, it was found that the periods ranged from 

 eighteen to twenty-seven days, the average being twenty days. This 

 was in green onions; according to Severin and Severin(i) the larval period 

 is prolonged into four or five weeks in seeded onions of the previous 

 year. Larvae of the later generations living in larger and more mature 

 onions seemed to take longer over that stage than the first generation. 



Pupation. When fully grown, the larva leaves the onion and enters 

 the soil to transform into the pupal condition. The exact position in the 

 ground varies but is generally at a depth of two or three inches and may 

 be close up against the onion or a short distance away. On pulling up 



