39 



food plant, and in some places fonr or live species living' in the 

 same locality : their specific characters lie in the male genital 

 organs, color of hody and wings and granulation of tegmina. It 

 is hard to conceive that these distinctions arose gradnally, and 

 were conserved by Xatnral Selection on account of their vital 

 importance, especially when we remember that the greatest mor- 

 tality is among the yomig, before these characters have ap- 

 peared. 



When considering the evolution of the characters of a group 

 of allied species, whether these characters be colors or struct- 

 ures, we must remember that they are all the results of physio- 

 logical processes, and that it is the changes in these processes 

 which constitute the real evolution ; the characters are the re- 

 sult of the physiological changes. 



There are certain cases where it appears highly probable 

 that internal physiological processes have had great eifect upon 

 the external characters. Kershaw^ in his work on the anatomy 

 of the '^Candle fly",* and in a subsequent paper, has shown 

 that in many families of Homoptera there is an enormous diver- 

 ticulum of the stomach, just behind the aesophagal valve, 

 which, lining up such suace in the thorax as is available, pro- 

 ceeds into the head. l\PPyropsrt)ictyopliorodelpliax, and sev- ^ 

 eral (perhaps all) other cases where the head is greatly elon- 

 gated, this diverticulum entirely fills this elongation**. Has 

 this structure been brought about because a slight increase in 

 size was of vital importance to the insect ; or through direct 

 use, such as pressure at all stages of development ; or by some 

 other means 't At present I incline to the second belief. 



The low percentage of nutriment in the liquid food of Ho- 

 moptera makes it necessary for the insects to pass great quan- 

 tities through their digestive organs, and the process of separ- 

 ating the wax and indigestible substances is of supreme impor- 



* A Memoir on the Anatomy and Life-History of the Homopterous 

 Insect Pyrops candelaria (or "Candle fly"). Zool. Jahr. XXIX, Abt. f. 

 Syst., pp. 105-124, Taf. 8-10, 1910. 



** The knowledge that this elongated head of Pi/rops is filled with 

 stomach may help to settle the much-controverted point as to the 

 luminosity of this structure. It has been suggested that the light is 

 due to bacteria, and as there is a luminous bacterium which lives in 

 the stomach of silkworm larvae, and makes the whole insect, quite 

 luminous, it is highly probable that bacteria in the stomach ot^Pi/rops 

 is responsible for the light seen on the head on rare occasions. 



