August 7, 19 19] 



NATURE 



445 



of examiners than the scientific outlook. In these 

 circumstances our author goes forward very 

 straightly. If he has to omit the tragedy of 

 Hudson's ending, he inspires us by a good map 

 of Amundsen's North-West passage, which is far 

 less known, because it belongs to recent history. 

 He cannot trifle with examiners by betraying 

 emotion at the " revolutions of the globe," but 

 he gives a good geographic account of the Ice 

 age in America (illustrated by a map of the 

 moraine-front and by an entirely inappropriate one 

 of medial and lateral moraines in Switzerland), 

 and he moves us effectively by his excellent choice 

 of illustrations. The South American section 

 opens an unfamiliar field before the reader. The 

 dangerous dunes on the MoUendo-Arequipa line, 

 and the fascinating glimpse of the Patagonian 

 ranges, may be cited from these interesting pages. 

 The unique nitrate-deposits of Chile (p. 258) might 

 have received fuller mention, since the material 

 is exfKjrted, not " for the preparation of various 

 chemicals," but for the increase of the food 

 supply of the nations, and its local refining and 

 foreign distribution are among the romances of 

 geography. G. .\. J. C. 



Carburettors, Vaporisers, and Distributing Valves 

 used in Internal Combustion Engines. By 

 Edward Butler. Second edition, revised and 

 enlarged. Pp. viii + 288. (London: Charles 

 Griffin and Co., Ltd., 1919.) Price 12s, 6d. net. 

 In the hundred or so pages of this work devoted 

 to carburettors the author has scarcely done 

 justice to the modern outlook, although he gives 

 an interesting description of various types. The 

 omission of the double Venturi form is remark- 

 able. No mention is made of aero work, yet it 

 was aviation which opened our eyes to efficiency. 

 Carburettor design is at last emerging out of 

 the embryo stage, and the rule-of-thumb method 

 of the inventor is giving place to scientific 

 measurement. Instead of a list of carburettor 

 patents, as given in the book, we should prefer 

 some experimental figures establishing the order 

 of merit of the different types and the justification 

 of the claims of the inventors. Thus, if a simple 

 carburettor of the Zenith or Claudel type does all 

 that is claimed for it, why go to the trouble of 

 fitting all sorts of extra air-valves? It would be 

 interesting to know why complicated designs can 

 persist side by side with simple ones. 



The remaining pages of the work are devoted 

 to vaporisers and injectors (suitable for stationary 

 engines under fairly constant load), and to the 

 consideration of types of valve gear. In this 

 connection we think that the advantages of the 

 sleeve valve gear are overrated. 



Treating of valve gear, we should have expected 

 some reference to the importance of turbulence of 

 the charge for high-speed work. Also the design 

 of induction pipes for multi-cylinder engines, and 

 the vibration of air in such systems (affecting 

 distribution and carburettor characteristics) are 

 matters of importance and deserve some comment. 



W. J. S. 

 NO. 2597, VOL. 103] 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications.] 



Wild Birds and Distasteful Insect Larvae. 



In Nature of July 24 is a letter from Dr. 

 Walter E. Collinge entitled " Wild Birds and Distaste- 

 ful Insect Larvae." This letter starts with a state- 

 ment with regard to the distasteful qualities of the 

 larva and imago of the currant moth (Abraxas grossu- 

 lariata, StephT), an insect which, of all others, has 

 probably been studied most in this connection. So 

 long ago as 1889 Prof. E. B. Poulton, in his classic 

 work on "The Colours of Animals," states 

 (p. 169) ; — " All observers agree that birds, lizards, 

 frogs, and spiders either refuse this species altogether, 

 or exhibit signs of the most intense disgust after 

 tasting it." 



Experiments of more recent date have done nothing 

 to refute this observation, and, what is more, the 

 larva of this moth differs widely from other Geometrid 

 or " Looper " caterpillars in making itself conspicuous 

 by means of a gregarious haibit and of the position 

 which is assumed when at rest, in contradistinction 

 to protective resemblance to twigs of trees, to the 

 eaten edges of leaves, etc., exhibited by most other 

 members of the family. 



Further, from his observations upon the case of 

 song-thrushes feeding their young uf>on the caterpillars 

 of this moth, and from the" fact that the latter, after 

 their destruction by the birds had ceased, were found 

 to be parasidsed. Dr. Collinge raises a question^ of 

 the possibility of the parasitised larvae alone being 

 rejected by birds. 



It is true that oarasitic insects, when depositing 

 their eggs in the interior of caterpillars, sometimes 

 cause an exudation of blood which dries over the 

 wound and produces a mark which acts as a warning 

 to other parasites not to oviposit in the same larva, 

 but, in the case of a spotted insect like the present 

 one, it is neither probable that such a mark would 

 be noticed by a bird, nor is it likely that all the cater- 

 pillars experimented with by entomologists were 

 similarly parasitised, even though insects which enjoy 

 protection through conspicuity of warning colours are 

 naturally subject to great persecution by their parasitic 

 enemies. 



A simpler and more logical explanation is applicable 

 to Dr. Collinge's observations. In the first place, no 

 insect, however well protected, is completely immune 

 from attack by enemies, and in times of stress birds 

 have long been known to subsist upon insects with 

 highly distasteful qualities. Of the eight birds men- 

 tioned by Dr. Collinge as containing currant-moth 

 larvae in their gizzards, the great tit, the house- 

 sparrow, and the cuckou are known to eat bees from 

 hives in winter or in spring, when food is very scarce. 

 The blue tit, flycatcher, and chaffinch are also ad- 

 dicted occasionally to this habit of eating an insect 

 with a powerful sting. 



A habit, probably of local origin, is exhibited by the 

 chaffinch when large numbers of humble-bees, and 

 even wasps, are attracted to the fragrant blossom of 

 the weening white lime-tree (Tilia petiolata) in 

 August. These insects are provided both with weapons 

 of defence in the stings of workers and queens, and 

 with warning colours in the shape of yellow and red 

 bands interspersed with black, but all these protec- 

 tions break down when they have sipped the nectar 

 from the flowers. 



