484 COMPARATIVE ANATOMY chap. 



like the bow of a fiddle over the projecting ribs of the upper wing. 

 In the Locustidce and Qryllidm only the males stridulate, by rubbing 

 the rough basal portions of their wing cases against each other. In 

 other insects sounds are produced by the rubbing together of other 

 parts of the body. 



4. In the singing Cicada also only the male can produce the well- 

 known shrill sounds. The sound apparatus is here somewhat com- 

 plicated. It consists of a pair of drum-skins (thin elastic extensions 

 of the cuticular skeleton) on the first abdominal segment, and of 

 strong muscles moving these skins. The abdomen filled with air acts 

 as a resounding apparatus. As a protection to the delicate drum-skins 

 folds of the thoracic and abdominal cuticular skeleton arch over them 

 from before and behind. 



The sounds produced by male insects are calls for attracting the 

 females. 



XI. Sexual Organs. 



The sexes are separate in all Antennata. A comparative study of 

 the sexual organs justifies us in giving the following general plan of 

 the sexual apparatus. It consists of a pair of germ glands (ovaries 

 in the female, testes in the male) which pass into paired duets, the 

 latter opening separately. The sexual glands and ducts appear, 

 as far as their ontogeny is known, to proceed from a paired mesoder- 

 mal genital rudiment. Ectodermal invaginations of the cuticle are 

 often connected with the ends of the ducts. 



Since the ducts of the sexual organs in the Protracheata are trans- 

 formed nephridia, we may perhaps infer the same for those of the 

 Antennata. There is, however, a considerable diff'erence in the two 

 cases, as the greater part of the ducts in Peripatus arise out of the 

 ectoderm, while in the Antennata, on the contrary, they come from 

 the mesoderm. But it must not be forgotten that in the Annulata the 

 greater part of the nephridium (nephridial duct) is of mesodermal origin 



Sexual organs, which are paired throughout (as in the plan above 

 sketched), are only found in reality in the Ephemeridce (Fig. 344, A). 

 In all other Antennata there are unpaired portions of the sexual appar- 

 atus arising in various ways. 



1. The two germ glands may fuse to form 1 unpaired germ gland, 

 while the ducts remain separate either throughout their whole length 

 or at any rate towards their ends, and always open externally through 

 separate paired apertures. Such cases are found in the Diplopoda 

 among the Myriapoda. 



2. The germ glands remain paired. The ducts remain paired for 

 the greater part of their course, only uniting to form a common 

 terminal portion. This is the case in all Antennata except the 

 Ephemerklm and the Diplopoda. 



