PHYSIOLOGY OF THE TNVERTEBRA7 A. 433 



certain parts * which are comparable to the amnion of the 

 Vertehrata. This amniotic investment is not, however, 

 universal among the Iiif^ecta, although it is present in the 

 Orthoptcra (Lihc/htlif), Diptcra, Lepidoptcra, Hymenoptera, 

 Colcoptera, and Hcmiptcra. 



As material agents in the propagation of the InsecM, the 

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

 and music. For instance, (i) in some Lepi(lo2')tcra'\ 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 

 Zibellidce 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 Lihdlula' 

 and Si/riphidcr." In other insects it is the males which have 

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

 Diptcra, Lcpidoptcra., Nntroptcra, &c., are said to be means 

 favouring copulation. The males of some Nciiroptera 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 Orthoptcra^ 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 longicorn beetles. 

 Reciprocating stimulatory friction of articulate parts to 



* The lamina of the sternal band, 

 t Argynnis, Zygnama, 3Iclitcca. 



2 E 



