170 WILLIAM BRODBECK HERMS 



The only special stuctures which might be considered as light 

 receptors are the unpigmented eye-like organs described by Lowne 

 ('90-92, pp. 71-72) at the extremity of the maxilla. These organs 

 have never to my knowledge been considered as visual organs, 

 and Weismann and Lowne evidently do not consider them as such. 



Concerning external segmentation Lowne ('90-92, p. 33) 

 states that ''Although it would appear at first sight perfectly easy 

 to count these segments, authors are by no means agreed as to 

 their number." The difference of opinion is due to invagination 

 at the anterior end and fusion at the posterior end. There are in 

 the two species considered in this paper, twelve obvious segments, 

 and when reference is made to the head-segment the first of these 

 twelve is meant. This first segment according to Lowne ('90- 

 92, p. 35) is composed of three metameres followed by three 

 thoracic and nine abdominal segments. Since there is only one 

 visible cephalic segment (distinguished by the maxillary hooks) 

 and since the last segment is a fusion of two, there remains the 

 obvious number twelve, not including the invaginated segment 

 of Newport (Lowne '90-92, p. 34) between the head and the first 

 thoracic segment. "When the maggot emerges from the egg 

 (Lowne, '90-92, p. 2) the parts destined to form the head in the 

 perfect insect are found deeply invaginated, and lie far back in the 

 thoracic region in front of the highly concentrated nervous system. 



"The central nervous system in the larva of the cycloraphic 

 Diptera generally (Lowne, '90-92, pp. 67-68) differs widely from 

 that of other insects in the close concentration of all the ganglia 

 in a single complex centre, which consists in part of the differen- 

 tiated nervous system of the larva, and in part of embryonic 

 structures destined to form the nerve centers in the nymph. Al- 

 though these parts are intermixed in a complex manner, the cellu- 

 lar elements of each are distinctly recognizable, and those which 

 are active in the larva undergo degeneration, like the rest of the 

 larval tissues, whilst those which are embryonic in type undergo 

 evolutions during the formation of the nymph." The central 

 nervous system which is devoid of the usual ventral nerve-cord 

 is comparatively short (Weismann, '64, p. 205), about one-twen- 

 tieth of the length of the body; in a larva 1.3 cm. long, it measures 

 only 0.74 mm. 



