CH. XV.] SPECIES OF DIPTEROUS INSECTS. 221 



sisted by a fluid that is probably secreted there, and 

 the author is therefore reduced to refer the effect to 

 molecular attraction only. It is also stated, that 

 when the foot of the fly is detached, a distinct fluid 

 trace will often be left by each individual hair, the 

 spotty pattern thus left on the glass appearing to be 

 of an oily character, for if breathed on it remains 

 after the moisture is evaporated. The contrary 

 opinion, although contained in a review of Mr. 

 Blackwall's Memoir above noticed, was evidently 

 written in ignorance of the subsequent observations 

 of that author contained in the appendix of the 

 volume in which it appeared, and in which several 

 facts are stated, which appear " quite inexplicable, 

 except on the supposition that an adhesive secretion 

 is emitted by the instruments employed in climbing;" 

 and it is subsequently affirmed, that careful and re- 

 peated examinations, made with lenses of moderately 

 high magnifying powers, in a strong light, and at a 

 favourable angle, speedily convinced Mr. Blackwall 

 that his conjecture was well founded, as he never 

 failed to discover "unequivocal evidence of its 

 truth." 



We have had a twofold object in thus setting be- 

 fore the reader, at considerable length, the various 

 opinions promulgated upon the subject, — the first 

 being occasioned by the interest attached to so 

 peculiar a phenomenon; and the second resulting 

 from a desire to show that, even in the commonest 

 insects, there are most ample materials of no ordi- 

 nary or uninteresting kind for the full exercise of 

 the mind of the ingenious observer of nature. It 

 will seem extraordinary, but it is nevertheless true, 

 that there is scarcely any domestic insect of whose 

 economy we are more ignorant than of that of the 

 Musca domestica. 



Other species exhibit another remarkable phe- 

 nomenon, which is almost without a parallel through- 

 out the countless myriads of the insect tribes. The 

 T2 



