PHYSIOLOGY OF THE INVERTEBRATA. 433 



certain parts * which are Comparable to the amnion of the 

 Vertebrata. This amniotic investment is not, however, 

 universal among the Insecta, although it is present in the 

 Orthoptera (Idbellula), Dipt era, Lepidoptera, Hymenoptera, 

 Goleoptera, and Semiptera. 



As material agents in the propagation of the Insecta, the 

 following may be mentioned : their odours, colours, dances, 

 and music. For instance, (i) in some Lepidoptera'] there are 

 two glands, situated near the opening of the vagina, which 

 secrete an odorous substance that excites copulation ; and 

 one could give many examples where odours play an 

 important part in the amours of various insects. (2) Female 

 LihelluloB and flies with bright metallic colours may often be 

 noticed reposing on plants in the sunshine, " attracting ever 

 and anon the attention of some passing male, who, staying 

 his course, remains for a while, as seized with an ecstasy, 

 suspended over their charms like a hawk marking his 

 quarry, and seeming as if dazed by the glow of pigment 

 beneath him. This is very characteristic of the lAhellulce 

 and SyriphidcB." In other insects it is the males which have 

 the gorgeous colours. (3) The aerial dances of certain 

 Diptera, Lepidoptera, Neuroptera, &c., are said to be means 

 favouring copulation. The males of some Neuroptera dance 

 and collect, and when joined by their attracted females they 

 pair. (4) Stridulation or. instrumental music is a character- 

 istic phenomenon in many insects. " The musical organs 

 sexually common in most beetles, butterflies, and moths, as 

 in a grasshopper genus, assume generally masculine differ- 

 entiation in the Ortlioptera, indicating dermal alteration and 

 induration ; they are either duplicate, paired, and similarly 

 situate as regards the bodies' median line, or their develop- 

 ment is single, as the alar organ of leaf-crickets, or quasi 

 unique, as in the family of bugs, and the longicom beetles. 

 Reciprocating stimulatory friction of articulate parts to 



* The lamina of the sternal band, 

 t Argynnis, Zygncena, MeKtcea. 



2 E 



