70 William B. H 



erms 



beyond are very chitinous, making a comparatively rigid shell, 

 which affords the optimum of protection. On the other hand, 

 the farther below the optimum, the less rigid the case, until at the 

 lowest extreme it is a mere flimsy covering. This shows that 

 here the least possible energy is expended, while the greatest 

 amount possible is stored up for the trying transformation from 

 larva to imago. 



Series J — Among the optimum series there were certain larvae 

 which were very slow in pupating. While others of the same 

 series pupated August i8 to 23, these particular ones had not by 

 August 27, the date on which they were weighed to ascertain the 

 reason for such tardiness. This weighing, in which 64 larvae were 

 taken, showed a mean of 33.59 mg., which in itself does not show 

 a high average [cf. series i, larvae). If we consider the lapse of 

 time since migration (twelve days) and the loss of moisture under- 

 gone during this time, we must see that this average is after all 

 high, and that these larvae were beyond the optimum weight, and 

 for this reason pupation was deferred. (For loss of moisture cf. 

 curve of growth for Lucilia caesar.) Of these 64 larvae 39 pupated 

 by September 9, giving us twenty days in the larval period against 

 a normal of about six days; the rest were dead. Furthermore, at 

 the present writing only 25 adults have emerged out of the 39 

 pupae. The mean of these adults is 21 mg., which shows pointedly 

 that the heaviest larvae had been eliminated or had dried out 

 sufficiently, resulting in adults very near the optimum weight 

 {cf. adults in series i). 



Series 8 — ^The sixty-five larvae in this series were picked from 

 the same fish upon which the optimum series was feeding, and con- 

 sequently could not have been poisoned. All the dead larvae with- 

 out exception were taken and weighed so that no selection was 

 possible. The larvae were carefully examined for injuries, but 

 only one was found to show an injury, the smallest of the lot 

 (25 rng.) In this case it was decided that death was due to this 

 cause and the weight was not included in the result shown. 



Thus we have what was expected, /. e., that continued feeding 

 without regard to the optimum would eventually result in death. 

 The mean for this series 46.73 mg. is extremely high and a com- 



