Species of Ptyclus 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 freces constructed by many 

 larva3, the calcium carbonate in the form of minute arra- 

 gonite crystals rubbed into its cocoon by Bomhy:': ncnstria, 

 or the hardened paste of calcium oxalate excreted and 

 made use of by the larva of Briof/aster 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 larva? are, however, a favourite food 

 with certain Hymenoptera, which pick out the larva? from 

 the spits and carry them off to be used as stores of pro- 

 visions for their larvae." 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. S. Morse of Salem, 

 Massachusetts. Even his account is but little known by 

 entomologists, because published in a somewhat iinusual 

 channel.* The general statement has always been that 



* At lirst in an elementary book on zoology : later in Appleton's 

 " Popular Science Monthly " for May 1900, p' 23. 



