234 Variation in the Mealworm, Tenebrio molitor 



Of those six conditions the fifth case, viz. the one spine lying exactly 

 in the median-line, was the most important for the object of these 

 investigations, because the one-spine character of larva and pupa is a 

 specific character of T. picipes, which is distinguished from all other 

 Tenebrio species in this particular aspect. The relatively larger number 

 of individuals, too, seemed to be an indication that the one-spine 

 character was not an anomaly of equal value with the five other cases, 

 but that I had here to deal with T. picipes. 



Of these one-spine larvae (with corresponding pupae) three genera- 

 tions were bred of which, however, all the larvae displayed the normal two 

 spines. Also the subsequent crossings of normal (/" x one-spine $ and the 

 reciprocal cross did not give any result. Later on, when examining again 

 a great number of larvae in search of single spined individuals, I found 

 cases in which the normal two spines were lying close beside almost in 

 contact with each other (Fig. 7, Nos. 1 and 2). Larvae of this kind gave 



pupae with only one spine, lying exactly in the median-line. When such 

 spines were examined under a powerful lens, it could be seen that the 

 spine was split; sometimes only at the top, sometimes as far down as 

 past the middle. Thus the so-called unpaired median spine appeared, in 

 several cases, to be paired. 



On the other hand, however, cases came under observation, where 

 the double nature of the single spine, even by magnifying it still stronger 

 (object-glass A, ocular 2), could not be ascertained, either in the larva or 

 in the pupa obtained from it. This individual, too, gave in cross-breed- 

 ing a negative result. 



8. Duration of the Larval Period. 



If the influence of the kind of food is left out of account, the develop- 

 mental stages as growth, pupation, the emerging of the beetles, the 

 copulation, the depositing and development of the eggs are in a high 

 degree dependent on the temperature. This strong sensibility to in- 

 fluences of temperature enables us to shorten the developmental stages 



