78 BRITISH GALLS 



millions of stout men — that is, more than the whole popula- 

 tion of China." It is generally held that Huxley's estimate 

 was below the mark ! Luckily, they have numerous enemies 

 which keep them well in check, but many species are often 

 serious pests in gardens. The generation with which 

 gardeners are mostly familiar is the parthenogeaetic young 

 produced by wingless females hatched from eggs laid in the 

 previous autumn. The production of living young by 

 females without male intervention may go on for several 

 generations. Later in the year, coincident with decreased 

 food-supply and lowering of the temperature, sexual insects 

 are produced, and the females deposit fertilized eggs, which 

 yield wingless females in spring. These apterous viviparous 

 females are often termed " mother queens " ; they are also 

 spoken of as " fundatrices." In the classified catalogue of 

 galls at the end of this book the expression " Aphis " refers to 

 the apterous viviparous female. Metamorphosis often takes 

 place amongst the parthenogenetic generations, and the 

 habits are very varied. It is the rule, however, that winged 

 forms appear when food is scarce, and wingless females are 

 usually the sexually perfect ones. Many Aphides produce 

 enormous quantities of a sweet sticky substance known as 

 " honey-dew," which is emitted through two tubes or 

 "cornicles" situated on the back. Kirby and Spence 

 remarked : " You have doubtless observed what is called 

 the honey-dew upon the Maple and other trees, concerning 

 which the learned Roman naturalist Pliny gravely hesitates 

 whether he shall call it the sweat of the heavens, the saliva 

 of the stars, or a liquid produced by the purgation of the 

 air. Perhaps you may be aware that it is a secretion of 

 Aphides, whose excrement has the privilege of emulating 

 sugar and honey in sweetness and purity. ... It issues in 

 liquid drops from the abdomen of these insects, not only by 

 the ordinary passage, but also by two setiform tubes placed 

 one on each side just above it." 



Probably the best-known of all aphid galls are those 

 caused by species of Chenites on the Common Spruce Fir. 



