Mar. 



Species of Pegomyia Mining Dock 



231 



seldom singly. They are laid in neat transverse rows on the undersur- 

 face of the leaf. In captivity, however, the eggs are scattered over both 

 surfaces of the leaf and are frequently laid singly instead of in groups. 

 The number of eggs occurring on a single leaf is surprising. As a rule, 

 one finds only 5 or 6 groups, but it is not uncommon to find more. In 

 one instance the writer found 20 groups of eggs on a single leaf, 65 eggs 

 in all. On another leaf, 16 inches long, he found 18 groups of eggs, 

 47 eggs in all. It is interesting to note that all the larvae from these 

 eggs did not mature within the leaf on which they were laid, but migrated 

 and started new mines on other leaves. The length of the e^gg stage is 

 given in Table I. 



Table I. — Incubation period of the eggs of Pegomyia calyptrata 



Kxperinient No. 



A112 



Am 

 A113 

 A117 

 A122 

 A139 

 A132 

 A116 

 As... 

 Ai34 

 A141 

 his.. 



Eggs 

 hatched. 



May 13 



May 14 

 ...do.... 



May 20 



May 22 



May 25 



May 30 



May 19 



May 17 



May 30 



June s 



June 26 



Length 

 of egg 

 stage. 



Days. 



LARViE. — The eggs hatch in from two to six days, and the young 

 larvae immediately enter the leaf, making small holes through the lower 

 epidermis. All the eggs of a single group hatch at the same time, and 

 the larvae feed in a common mine, which is at first linear. The larvae 

 mine side by side, progressing only in a forward direction. They keep 

 close together, and all change their direction of mining at the same time, 

 leaving behind them a short linear path (Pi. 28, D). In about a day, 

 although no definite time can be set, the larvae begin to enlarge their 

 mine laterally, forming a blotch. They still remain in a common mine, 

 but separate in different directions! It is not an unusual sight to see 

 several such blotches on a leaf. Each represents a number of larvae 

 that have hatched from a single group of eggs. These increase in size 

 until they interfere with each other, and a large blotch is produced, 

 covering the entire area of the leaf. Many of the larvae are naturally 

 forced to abandon their mines and form new ones in other leaves. The 

 presence of nearly mature larvae in small blotch mines is an indication 

 that they have entered fresh leaves (Pi. 28, H). 



