52 DIPTERA OF AUSTRALIA, 



in which the insect ■will eventually alight. A great number, 

 if not the majority, of species have a peculiar proclivity for 

 spiders'-webs, and this is particularly noticeable in the numerous 

 caverns, and under the overhanging rocks, on the Blue Mountains, 

 and elsewhere, where the spiders have from crag to crag stretched 

 their long threads of silk. In such secluded nooks and corners 

 Cecidomyida? may be seen sometimes in tens of thousands 

 suspended closely side by side along the lines, evidently reposing, 

 but commencing immediately, when approached, a peculiar oscil- 

 lating movement which tends to render the insects themselves 

 at the best only indistinctly visible on account of the x'apid tremor, 

 and very similar to the vibratory motion produced by an alarmed 

 spider from the centre of its geometrical snare, or the long-legged 

 Tipulidaj when at I'est on a wall. In all these cases the motion 

 seems to be equally a stratagem employed for protection, — in 

 other words, a manceuvre whereby to escape the observation of 

 their enemies. 



It is worthy of notice, too, that the Cecidomyid?9 perfectly 

 ignore both the purpose of the cobwebs and the nature of their 

 dangerous proprietors ; never once have I, or has Mr. Masters, 

 observed any of their remains ensnared, nor do the spiders appear 

 to trouble themselves one iota about these small flies, which 

 suddenly take wing on the least apprehension of danger. Not- 

 withstanding the insignificant size of the generality of the gall- 

 gnats, if a considerable number of festoons be disturbed, the 

 hitherto silent cave soon reverberates with the harmonious hum 

 produced by the vibi'ation of thousands of little wings. 



I have but rarely seen a Cecidomyid walking, that is, quietly 

 travelling without using the wings ; or standing at rest on a solid 

 or hard foundation, if there was the smallest film of cob-web on 

 which to hang, within reach. 



The Cecidoniyidse,* or gall-gnats, as their name im])]ies, attack 

 plants, thereby causing excrescences or distortions of the particular 



* From KTjKis, a gall ; /xvTa, musca, a fly. 



