COLORATION IN POLISTES. 71 



Evidence tending to confirm these last three laws is further elabo- 

 rated in the following paragraphs : Chief among the environmental 

 influences which play a role in the color differentiation of Polistes are 

 temperature and humidity taken in conjunction, although temperature 

 appears to affect mainly the amount of the pigment, and moisture its 

 kind and degree of suffusion. 



The colder the climate, and the more prolonged the pupal period, the 

 greater the amount of pigment produced ; but whether that pigment 

 shall spread uniformly over the surface or be restricted to definite 

 areas depends also on other conditions, one of which appears to be 

 humidity. This may be imagined to affect the mobility of the pig- 

 ment as it is being deposited. When the moisture is abundant it is 

 safe to suppose the pigment to be more mobile, and the chitin, hard- 

 ening more slowly, allows it to spread farther from its center of dis- 

 tribution. When slight in quantity the pigment does not spread so 

 far ; so, with the same amount of pigment deposited, we may have 

 under dry conditions numerous small dark areas interspersed with 

 large areas of hypodermal yellow i. 6'., P. aurifcr or under moist 

 conditions a uniformly-colored light species, such as P. carolinus of 

 Florida. 



Moisture also appears to affect the color of the pigment. Abun- 

 dance of moisture gives the pigment a redder color, scarcity a duller 

 brown color (cf. P. aurifer and P. carolinus}. Further, in P. ftavus 

 we have the apparent effect of a hot, dry climate on a form allied to 

 P. texanus, into which it passes by transitions, and in P. navaioe and P. 

 carnifex the effect of similar conditions on the canadcnsis type. Finally 

 the increase in depth of melanism as 7/e go northward is evidence for 

 the effect of temperature, but the fact that the melanism is more 

 marked along the coast than inland indicates that moisture is also a 

 factor here. On the eastern portions of both the old and new worlds 

 the melanism is, as it were, impressed on a rich red-brown form that 

 has developed in the humid regions of the East Indies and the West 

 Indies and Florida, which is practically one with the latter so far as 

 climatic conditions are concerned. In the western portions the melan- 

 ism affects the type peculiar to the arid zones of southwestern United 

 States and northern Africa. 



This hypothesis is further borne out by observation in individual 

 variation in a number of instances, and in a striking manner also by 

 the parallel types of coloration observed in genera related to Polistes. 



