_A. Levana_ may produce not only two but three broods in the year, and may, therefore, be said to be _polygoneutic_.[16] One winter brood alternates with two summer broods, the first of which appears in July, and the second in August. The latter furnishes a fourth generation of pupæ, which, after hibernation, emerge in April, as the first brood of butterflies in the form _Levana_. I frequently placed pupæ of this fourth brood in the hothouse immediately after their transformation, and in some cases even during the caterpillar stage, the temperature never falling, even at night, below 12° R., and often rising during the day to 24° R. The result was always the same: all, or nearly all, the pupæ hibernated, and emerged the following year in the winter form as perfectly pure _Levana_, without any trace of transition to the _Prorsa_ form. On one occasion only was there a _Porima_ among them, a case for which an explanation will, I believe, be found later on. It often happened, on the other hand, that some few of the butterflies emerged in the autumn, about fourteen days after pupation; and these were always _Prorsa_ (the summer form), excepting once a _Porima_. From these experiments it appeared that similar causes (heat) affect different generations of _A. Levana_ in different manners. With both summer broods a high temperature always caused the appearance of _Prorsa_, this form arising but seldom from the third brood (and then only in a few individuals), while the greater number retained the _Levana_ form unchanged. We may assign as the reason for this behaviour, that the third brood has no further tendency to be accelerated in its development by the action of heat, but that by a longer duration of the pupal stage the _Levana_ form must result. On one occasion the chrysalis stage was considerably shortened in this brood by the continued action of a high temperature, many specimens thus having their period of development reduced from six to three months. The supposed explanation above given is, however, in reality no explanation at all, but simply a restatement of the facts. The question still remains, why the third brood in particular has no tendency to be accelerated in its development by the action of heat, as is the case with both the previous broods?