THE MEDIAN SEGMENT 



163 



represented by the anal or lateral plates. It thus appears that even 

 in the embryo condition of the more generalized winged insects, the 

 number of uromeres is slightly variable. 



We have designated the abdomen as the urosome ; the abdominal segments 

 of insects and other Arthropods as uromeres, and the sternal sclerites as nro- 

 sternites, farther condensed into urites. (See Third Report U. S. Entomological 

 Commission, 1883, pp. 307, 324, 435, etc.) 



The reduction takes place at the end of the abdomen, and is 

 usually correlated with the presence or absence of the ovipositor. 

 In the more generalized insects, as the cockroaches, the tenth seg- 

 ment is, in the female, com- 

 pletely aborted, the ventral 

 plate being atrophied, while 

 the dorsal plate is fused dur- 

 ing embryonic life, as Cholod- 

 kovsky has shown, with the 

 ninth tergite, thus forming 

 the suranal plate. 



In the advanced nymph of 

 Psylla the hinder segments of 

 the abdomen appear* to be fused 

 together, the traces of segmenta- 

 tion being obliterated, though the 

 segments are free in the first stage 

 and in the imago (Fig. 178). It 

 thus recalls the abdomen of 

 spiders, of Limulus, and the pygi- 

 dium of trilobites. 



FIG. 178. Nymph of the pear tree Psylla, with its 

 glandular hairs. After Slingerland. Bull. IMv. Ent. 

 I". S. Dep. Agr. 



The median segment. There has been in the past much discussion 

 as to the nature of the first abdominal segment, which, in those 

 Hymenoptera exclusive of the phytophagous families, forms a part 

 of the thorax, so that the latter in reality consists of four segments, 

 what appearing to be the first abdominal segment being in reality 

 the second. 



Latreille and also Audouin considered it as the basal segment of the abdo- 

 men, the former calling it the "segment me'diaire," while Newman termed it 

 the " propodeum." This view was afterward held by Newport, Schiodte, Rein- 

 hard, and by the writer, as well as Osten Sacken, Brauer, and others. The 

 first author to attempt to prove this by a study of the transformations was New- 

 port in 1839 (article "Insecta"). He states that while the body of the larva 

 is in general composed of thirteen distinct segments, counting the head as the 

 first, "the second, third, fourth, and, as we shall hereafter see, in part also the 

 fifth, together form the thorax of the future imago" (p. 870). Although at first 

 inclined to Audouin's opinion, he does not appear to fully accept it, yet farther 



