Morphological Variation and Its Causes in A. tigriiium ii 



a regular but light feeder and had attained its slender and grace- 

 ful proportions by slow but prolonged growth. 



The effects of a continued free-swimming habit, as opposed to 

 quiescence at the bottom, are, when the contrast can be well ob- 

 served and the habit is prolonged, hardly less marked than the 

 effects of slow growth. Specimens raised in the same aquarium 

 or in similar aquaria side by side, with all conditions as uniform 

 as it is possible to make them, seldom fail to furnish striking ex- 

 amples of broad-headed, short-bodied, and short-tailed types 

 which are habitually found at the bottom, while others, slender 

 and elongated, are free swimmers and maintain themselves in 

 almost as continual suspension and motion as does a gold fish. 

 Figure i, plate IV, shows a young adult from such a larva, 

 greatly shrunken by preservation, it having measured, when 

 chloroformed, 19.7 cm. in length and but 1.8 cm. in width of 

 head. It was the last to metamorphose out of several hundred 

 specimens which I fed with meat in a small enclosure in a pond. 

 Together with several other specimens of the same type, it 

 maintained the free-swimming habit all summer in its narrow 

 quarters, while the majority of the specimens, lying lazily at the 

 bottom, metamorphosed much earlier, as short-tailed, robust indi- 

 viduals. Out of this same series the three most slender larvae did 

 not metamorphose at all. In some instances this type of differ- 

 entiation may be found working on the wholesale in nature. Thus 

 two summers since I had occasion to follow closely the develop- 

 ment of larvae from April to September in a small pond. A few 

 specimens were netted almost daily and again returned to the 

 pond. Food (daphnids and chironomous larvae) was very abun- 

 dant; the number of larvae was limited, and growth of all was 

 very rapid. Yet despite the uniformity of these favorable condi- 

 tions, the larvae soon began to split up into two noticeably dis- 

 tinct groups, the one of unusually compact proportions, the other 

 of uniform intermediate build, such as is most commonly met 

 with. Figure i. plate I, and figure 2, plate II. show side and dor- 

 sal views of the robust type, the adults from the same larvae being 

 shown on plates III and IV. Figure 3. plate I, shows a side view 

 of a free-swimming larva. It is but moderately slender, or rather 



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