348 FOREST ENTOMOLOGY. 



" The parts chosen for egg-laying are most commonly about two 

 years of age and more, with twigs up to three inches in diameter. 



" Life-history. 



" The female does not hore (her ovipositor is too weak), hut lays 

 her eggs in chains or rows on the bark. ' The larvae bore into the 

 bark, but by the time they enter below it the cambium has already 

 made the first layers of wood. Owing to the irritating presence of 

 the larvae, the cambium gives rise to irregular streak- like growths, 

 through which the pests make longish excavations ; the galleries are 

 irregular. Between the larval galleries the wood is normal in con- 

 dition. It is possible that the larvae may not really need to bore in, 

 as the activity of the cambium, with its wood formation, may be 

 sufficient to enclose them. 



" For a time the bark stretches, accommodating itself to the in- 

 creased thickening, so that only spindle-shaped swellings show ; but 

 ultimately it ruptures and hangs down in shreds. For pupation the 

 full-fed larva betakes itself to the periphery, pupation taking place 

 under only a thin external layer, which is easily broken through by 

 the two horns of the pupa. The empty pupa skin, with its ' forehead 

 horns,' may be seen jutting from the round hole until the weather 

 removes it. The bark from which flies have issued may be seen 

 riddled with small holes. 



"There is one generation in the year, the larvae tunnelling from 

 June or July till the next April, May, or June. 



" Measures of Preservation and Remedy. 



"1. Cut off and burn infested shoots before issue of the brood. A 

 very observant forester will recognise the swelling before rupture of 

 the bark, and should remove it. Another sign of larval presence is 

 the poor leafage. The cut-away parts must not be left lying, else 

 development and issue may be completed. 



" 2. Streak over with tar the places attacked, as though the pupa 

 may push its way through, and the fly will be caught in the sticky 

 material." 



Cecidomyia (Rhabdophaga) heterobia, Lw. 



This species, which gives rise to the malformations as seen in fig. 

 322, is, I have found, often very closely associated with G. marginem- 



