Species of Ptyelus in British East Africa, 227 
nutriment contained in the sap, so that a great quantity 
must pass through the body of the insect in order to yield 
a sufficient supply of food. Analysis of sap drawn direct 
from the tree as compared with that of the fluid which 
has passed through the body of the insect might well 
yield interesting results bearing upon the physiology of 
insect nutrition. 
The frothy covering is a good example of the utilization 
of an excretory substance for the purposes of defence, entirely 
analogous to the covering of fseces constructed by many 
larvee, the calcium carbonate in the form of minute arra- 
gonite crystals rubbed into its cocoon by Lombyx neustria, 
or the hardened paste of calcium oxalate excreted and 
made use of by the larva of Hriogaster lanestris. 
Dr. David Sharp (1. c. p. 578) makes the following state- 
ment concerning the protective value of the froth :— The 
frog-spit is considered by some naturalists to be a pro- 
tective device; the larvae are, however, a favourite food 
with certain Hymenoptera, which pick out the larvee from 
the spits and carry them off to be used as stores of pro- 
visions for their larve.”’ It is strange that, Dr. Sharp 
should quote this observation as if in refutation of the 
opinion that the secretion is protective. I do not know 
of a single naturalist, except the late Dr. Haase, who holds 
or has held that any defence of this kind is effective 
against all enemies and that universal immunity is thereby 
conferred. Such a conclusion is unthinkable, and yet it 
is the only conclusion controverted by Dr. Sharp’s state- 
ment. The category of special defences to which belongs 
the covering of froth involves conspicuousness and easy 
capture by special classes of enemies. But can it be 
doubted that the adaptation confers nevertheless a balance 
of advantage in the struggle for existence? The justifica- 
tion of any such doubt requires evidence on a very different 
scale from that brought forward by Dr. Sharp. 
The method by which the froth is produced has been 
misunderstood and erroneously described probably by every 
author who has written upon the subject, until it was 
studied by my friend, Professor E. 5. Morse of Salem, 
Massachusetts. Even his account is but little known by 
entomologists, because published in a somewhat unusual 
channel.* The general statement has always been that 
* At first in an elementary book on zoology : later in Appleton’s 
‘ Popular Science Monthly ” for May 1900, p. 23. 
