74 
Araneidea, and this is a most worthy work crowning the edifice of his 
labours. It seems ungracious, therefore, to say anything that should 
for a moment lessen our appreciation of it; but it certainly would 
have been a great advantage to the students of this volume if Mr. 
Blackwall had said something of the diagnoses of the different genera, ” 
and particularly had given some collective idea of thei, as they 
occurred, under each family. At present we have to turn over about 
380 pages if we would know what genera have a place in our Fauna. 
As only one order is treated, it is also desirable that some notice 
should have been taken of the other orders forming the great class 
Arachnida. There is a difference of opinion on this subject, and we 
should all have been much interested if one so competent as Mr. 
Blackwall had put us in possession of his views respecting the 
correlation of the spiders to the other groups of its class. 
In the last and in a previous part of the Linnean ‘Transactions’ 
are some beautiful plates representing highly magnified views of the 
under surface of the tarsi of insects. These are illustrative of a paper, 
by Mr. Tuffen West, “On the Foot of the Fly” (vol. xxiii. p. 393). 
Mr. West considers the pressure of the atmosphere the main agent by 
which a fly is enabled to adhere to smooth surfaces, and that access 
of air is prevented by the minute quantity of moisture which exudes 
from the expanded tips of the “tenent” hairs. This alone, however, 
Mr. West does not seem to think sufficient; he would supplement it 
by what he calls the “grasping” power of the foot, “by molecular 
attraction, and doubtless by other agents still more subtle, with which 
we have at present scarcely any acquaintance.” 
Mr. Blackwall,* who commenced his observations thirty years ago, 
thinks that this hypothesis is “absolutely irreconcilable” with the 
result of his own observations and experiments. He says that they 
traverse the upright sides of the dome of the exhausted receiver of an 
air-pump as long as their physical energy remains unimpaired, and 
occasionally remain fixed to the glass after having lost the power of — 
locomotion, a circumstance which he’ considers can only be explained 
by admitting the adhesive property of the fluid emitted from the 
extremity of the papilla on their pulvilli, Mr. Blackwall says the 
“Spiders that are provided with tarsal brushes run with celerity on 
the vertical surfaces of highly polished bodies, as those instruments 
consist of numerous appendages slightly curved downwards and some- 
what enlarged towards their extremity, which is densely covered on its 
* Proc. Lin. Soc. Zool. vii. p. 159. A recent paper, not yet published, has been 
read at the Linnean Society, in which these statements are re-aflirmed. 
