STUDIES OF VARIATION IN INSECTS 293 



largest is that condition in which all the spots are present, dis- 

 tinct and well defined (Class J ; Class I is partly only a sub- 

 division of Class J). From the series it is apparently not pos- 

 sible to suggest the probable tendency of the pattern's evolution ; 

 perhaps a larger series would throw more light on the matter. 

 The strikingly large range and pronounced character of the 

 variations in this short series of specimens cannot but be inter- 

 esting and suggestive to students of the significance of variations. 



Variations in the Prothoracic Pattern of Corisa sp. (Water- 

 boatman). — The Corisidae, water-boatmen, are aquatic Hemi- 

 ptera, common in all fresh-water ponds and stream pools. The 

 back is patterned by fine lines, more or less wavy, of pale color on a 

 dark ground so as to produce the impression of a fine mottling. 

 The combination of color and patterning is such as, without 

 doubt, to make the insect very inconspicuous in the water. 

 Indeed to the limited vision of its enemies, such as other pre- 

 daceous aquatic insects, fishes, etc., it ought to be nearly indistin- 

 guishable when at rest, and thus only 

 betrayed by its movements. The 

 pattern on the dorsum of the pro- 

 thorax consists of (usually) thirteen 

 or fourteen fine transverse lines (fig. 

 58). The variation in the number 

 of these lines was determined in a 

 lot of 502 individuals taken by a fig. 58. The water boat- 

 single sweep of the collecting net man, Cor/5a sp., showing trans- 

 £ J *u C+^^-f^^^ TTr,; verse wavy lines forming the 



from a pond on the btanford Uni- / 



^ prothoracic pattern. 



versity campus. These insects have 



an incomplete metamorphosis and in their development from 

 young to aduh, exposed for this whole time to the direct influ- 

 ence of the environment which usually remains that of the 

 whole adult life as well, they undergo whatever changes take 

 place between their condition at time of hatching and that at 

 maturity. The advantage or worth of the color-pattern is 

 indeed constantly subjected to test during the whole life after 

 hatching. 



The classes of variation in number and character of these 

 lines constituting the prothoracic pattern, and their frequencies 



