254 FOREST ENTOMOLOGY. 



especially ants, wbicli not only sip the juices but extract the honey- 

 dew from the aphis by squeezing it from the cornicles. I have often 

 observed ants doing this on the aphides on elder. 



The honey-dew is very injurious to vegetation, for not only does it 

 have a tendency to choke up the stomata or breathing-pores of the 

 leaves, but the injury is still furthur increased by the honey -dew 

 forming a lodgment for soot, dust, and dirt. 



The tail, though not an important appendage in the discrimination 

 of genera or species, must not be forgotten in generic distinction, inas- 

 much as it is long in some, and short or entirely absent in others. 



The life-history of an aphis presents many interesting points, either 

 considered biologically or as evidenced by the mysterious appearance 

 and disappearance of the so-called "blight" or " green-fly." 



The aphis generally appears in numbers simultaneously with the 

 green foliage.- The first brood of the season originates either from a 

 female which has hibernated during the winter months, or from eggs 

 — that is, generally true eggs, resulting from male fertilisation. In 

 the former case, the "stem-mother" {Stamm-midter), as the Germans 

 call her, very much Tesembles the queen wasp, inasmuch as she is the 

 mother or founder of a whole summer's colony. As a rule, she is 

 considerably larger and less active than any of her offspring. 



The eggs of Aphididae may sometimes be seen in autumn as small 

 white specks on the stem, though generally they are green-brown or 

 bottle-green. They become dark in winter, and, of course, are not so 

 conspicuous as in the autumn months. 



The brood arising from eggs which had been deposited in autumn, 

 or from the "stem-mother" in spring, give rise to insects which are 

 generally all females. These may be found in two forms — viz., 

 winged and apterous. In many genera, when but a few days old, 

 both forms are capable of giving birth to living young. Each in- 

 dividual insect produces from 90 to 100 progeny, and so on until 

 about twenty generations are produced in a single season. Hence it 

 will be seen by a progressive calculation that the numerical strength 

 of the progeny produced by a single insect certainly becomes ap- 

 palling, and the number just given is under rather than over the 

 average. 



In order to realise the numerical strength of aphis reproduction, 

 Professor Huxley made some remarkable calculations, and has there- 

 fore put them in his own inimitable way. He calculated that if an 



