552 



A Cause of Larch Canker. [dec, 



themselves to be crushed to death without making any 

 attempt to fly. 



The formation of the insect may be seen from the illustrations. 

 Fig. 2 shows a specimen bearing on its thorax two pairs of cases 

 containing the wing membranes before these have unfolded ; 

 this condition is equivalent to that of the pupal condition of 

 insects with a complete metamorphosis. The larva lacks these 

 wing-cases. The wings are held folded one pair over another 

 and sloping peculiarly along the back, so that in side view a 

 large one only is observable, while viewed from above there is a 

 curious shield-like outline as seen in Fig. i. These insects 



fig. I. — The fungus fairy fly (Caeci- fig. 2. — Pupa of Caecilius flavidus, 

 Hits flavidus) in resting position much magnified, 



(magnified). 



can be identified by the nervures or veins of the wings ; the 

 neuration is irregular and not abundant. The eggs are laid 

 on bark, leaves, fungi, decaying matter, &c. 



I have found very large numbers of these insects in the 

 " witches' brooms " of the birch and also among samples of larch 

 canker. Other cankerous states of growing wood also yield 

 them in abundance. In roaming about among their favourite 

 haunts the Psocids get densely covered with the fungus spores 

 and so convey them to any fresh districts which they may 

 visit. These Psocidce may live on the fungus and its spores 

 (certain species, for example, live on rust fungi) but do harm 

 by carrying the spores about and so spreading the disease to 



