340 Annals Entomological Society of America [Vol. X, 



series show very clearly that in the Neuroptera (among which 

 are found certain forms which have departed but little from the 

 ancestral condition of the Mecoptera) instead of merely one 

 type, there are at least two developmental tendencies, the one 

 leading to a retention of a shorter type of head such as that of 

 the Neuropteron shown in Fig. 1, while the other leads to the 

 formation of a more elongate type of head, such as that of the 

 Neuropteron shown in Fig. 4.. These two tendencies are 

 carried over, or re-appear, in the Mecoptera, which are descended 

 from Neuropteron-like forebears. Thus the short-headed type 

 is retained in such Mecoptera as that shown in Fig. 2 (which, 

 however, exhibits a slight tendency toward a narrowing and 

 lengthening of the lower portion of the head), while the ten- 

 dency toward the formation of the elongate type of head appears 

 again in such Mecoptera as that shown in Fig. 5. Similarly, 

 in the Diptera, which in turn are derived from Mecopteron-like 

 forebears, the same two tendencies reassert themselves, some 

 of the Diptera having retained the short-headed type, as shown 

 in Fig. 3, while other Diptera, such as the one shown in Fig. 6, 

 have developed the elongate type of head. 



It might be argued that a similar mode of life, or similar 

 "environmental" conditions might cause a marked similarity 

 in outline in the heads of the insects in question, and that this 

 similarity is therefore due to a convergence — or rather to a 

 parallelism of development. However, the marked morpho- 

 logical similarity in a series of structures taken from widely 

 separated parts of the body (e. g. mouthparts, thoracic sclerites, 

 legs, terminal abdominal structures, etc.) and the marked 

 resemblance which extends even to the more minute details, 

 and in parts which are not much used, or are not of vital impor- 

 tance to the organism, would preclude the possibility of a mere 

 parallelism of development — which might possibly be the 

 case if we were dealing with a single set of structures alone; 

 but to argue that a parallelism of development has brought 

 about the similarity in structure between all of these parts 

 of the body in the series, is demanding too much of chance and 

 the "law of probability." 



While claiming that the series of insects represented in Figs. 

 1, 2 and 3 and the series represented in Figs. 4, 5 and 6, to all 

 intents and purposes serve to illustrate what has actually 



