ZOOLOGY AND BOTANY^ MIOEOSCOPY, ETC. 47 



mental fold, and, therefore, a derivate of tlie outer integument of tlie 

 body. This process of differentiation in the Perlidse may be regarded 

 as typical of several groups of insects, complications notwithstanding. 

 The generative organs of insects may, therefore, be regarded as 

 formed from two elements which are morphologically distinct — the 

 primitively internal paired structures (testes and vasa deferentia, 

 ovaries and oviducts) and tegumentary structures. In the least 

 differentiated groups (as in lower animal forms) the latter are only 

 represented by the genital orifices, and, consequently, the whole appa- 

 ratus is paired. The paired parts may become secondarily unpaired 

 in four different ways — there may be the invagination of a common 

 tegumentary portion, or the internal ducts may anastomose and fuse 

 proximally to their origin, or these two processes may take effect 

 together, or, finally, one of the symmetrical parts may become 

 superfluous and reduced. 



Thoracic Musculature of Insects.* — C. Luks investigated the 

 thoracic musculature of insects of every group, except, unfortunately, 

 the Thysanura and Collembola. He finds that the wing-muscles appear 

 to have developed along two lines ; in one, the indirect flying muscles 

 were almost completely aborted, while in the other they were developed 

 at the expense of the direct muscles. In close connection with the 

 development and modification of these muscles is the extent of con- 

 centration of the rings of the thorax and the size of the wings. In 

 the Orthoptera all the three segments of the thorax are freely movable 

 on one another, while in the Coleoptera only the prothorax is so movable. 

 In the Lepidoptera the prothorax loses its mobility, though retaining 

 its distinctness, while in the Diptera and Hymenoptera the whole 

 region is converted into a firm thoracic apparatus, to which, in the 

 latter, the first abdominal segment also becomes applied. As Graber 

 has shown, we observe that in insects which, by other points in their 

 organization intimate that they are more highly developed, one pair 

 of wings tends to become aborted, as is seen in the Coleoptera, 

 Diptera, and even Lepidoptera where the hinder pair of wings often 

 become united with and share in the movement of the anterior pair. 



Early Developmental Stages of Viviparous Aphidesf — L. Will, 

 after noticing the results of earlier observers, comes to his own 

 observations on the development of the ova ; to examine the ovaries a 

 fresh individual was opened on the slide in a weak salt solution, or in 

 iodized serum ; acetic acid was occasionally added, but this is a 

 reagent which must be used with caution. Water at about 70° C. 

 was found the best killing agent. Sections were largely made, and 

 these were coloured with borax-carmine and hsematoxylin. 



The common oviduct opens on the ventral surface and in the 

 median line, a little in front of the anus; as is well known, the 

 seminal vesicle and cement-glands which are present in the oviparous 

 are wanting in the viviparous forms. Connected with the oviduct 

 by special canals are a number of ovarian tubes, the walls of which 



* Jenaisch. Zeitschr. f. Med. u. Naturwiss., xvi. (1883) pp. 529-52 (2 pis.), 

 t Arbeit. Zool. Zoot. Inst. Wiirzb., vi. (1883) pp. 217-58 (1 pi.). 



