Z'2- J. H. Pozvcrs 



head as well as the general form. But it is not difficult in many 

 cases to trace the results of the separate factors. 



On plate III are shown, in figures 4 and 5, the Hfeads of a larva 

 and an adult which present proportions very unlike the heads of 

 ordinary specimens, at least as the species occurs in the West. It 

 will be noticed tliat these heads are excessively high or thick; 

 that they are as short as they are high, and that the gape is so short 

 as to render the mouth very small. Such heads exaggerate the 

 type which Cope has figured as the eastern variety of the species. 

 Could I have figured the entire animals, it would have been ob- 

 served that, viewed dorsally, these heads seem positively dimin- 

 utive for A. tigrimcm. Several adults that I now possess, are even 

 smaller-headed still, and had they been obtained from a less 

 known habitat few systematists v/ould, I think, have classified 

 them as A. tigriniwi, more especially as their colors are as unusual 

 as is their type of form. It may be interesting in passing to con- 

 trast these extremely short, thick, small-mouthed heads with an 

 opposite type which I have figured on the same plate. I will ex- 

 plain this head, with its enormous gape of mouth later. I will 

 here note only that the entire length of this animal was (in form- 

 alin) but 22.3 cm., while the short-headed adult was 23.2 cm. 

 long, and the larva 25.4 cm., the last two being measurements 

 from living animals. 



As to the cause of development in the case of these small, yet 

 thick heads, I have never been able to produce them by laboratory 

 experiments ; but have secured nearly two score of them by plac- 

 ing animals in a reservoir where they developed for a full year 

 under conditions that I could fairly well observe. The smallest 

 heads have been produced only in years when the food supply 

 was very light for at least the first half of the season. The water 

 was relatively clear, allowing the animals to feed by sight. ^ No 

 daphnids and few other small, free-swimm.ing organisms were 

 present for food until late in the season, when water-boatmen 

 became abundant. The food consisted of insect larvae, especially 

 of dragon fly larvae, which became in the latter part of the season 



'The size of the eyes varied, but in some specimens it was noticeably 

 greater than in animals of equal size grown in muddy ponds. 



228 



