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June, 1919 Bulletin of the Brooklyn Entomological Society 95 
holometabolous group, as would be expected if holometabolism 
originated in the warmer climates and enabled its possessors to 
penetrate and establish themselves in a region of rigorous 
climate; so that instead of showing that holometabolism origi- 
nated in a rigorous climate and that it was caused by cold, Hand- 
lirsch’s figures would show that holometabolism probably origi- 
nated in the warm climatic conditions in which insects were de- 
veloped, and enabled those forms in which it occurred, to pene- 
trate the regions of more rigorous climate. 
If metabolism were brought about by the insertion of a pupal 
stage due to the influence of cold, we would expect that in 
temperate climates most holometabolous insects would spend the 
winter in the pupal stage, whereas, on the contrary, most of them 
pass the winter as larve and adults (or even in the egg stage).* 
The pupal stage is a helpless one, and if there were a tendency 
on the part of some holometabolous insects to enter the pupal 
condition at about the time of the onset of cold weather, when 
parasites and other enemies were not active, this feature might 
confer some degree of protection upon the insects in which the 
tendency occurred, and thereby tend to preserve this trait; 
although other forms having developed other means of protect- 
ing the helpless pupz, might pass the winter in other stages of | 
development. It must be borne in mind, however, that the inser- 
tion of a pupal stage in the life history of insects is not the 
chief feature in the development of holometabolism; but the 
production of a larval form differing greatly from the adult was 
*T think that the cause of the insertion of a pupal stage can be better 
explained thus. When the immature stages of an insect are essentially like 
the adults (as in lower insects), there is no necessity for the insertion of 
a pupal stage, to assume the adult form; but when the immature stages 
and adults have gradually come to differ so greatly from each other (as 
each develops along its own line of specialization) that a great change 
must be undergone by the immature form in assuming the adult condition, 
this necessitates the insertion of a pupal stage, or “ making over ” period. 
It has been suggested that the gradual changes taking place through sev- 
eral moults in the individual development of the lower insects, have, so to 
speak, been concentrated in the pupal stage of insects with complete meta- 
morphosis; but I do not think that this is an adequate explanation for the 
origin of a pupal stage, even if it were found to be the case that the lower 
insects moult more frequently than the higher forms do. 
