THE ARTHROPODA. 233 



There is some reason to think that the antennse of Insects 

 are the seat of the olfactory function, but no certain infor- 

 mation of this head has been obtained. The very fine setae 

 to the bases of which nerves can be traced, which abound on 

 the antennary organs of Insecta and Crustacea^ but are found 

 in other regions of the body, are probably partly tactile and 

 partly auditory organs. 



As a general rule, all the muscles of the Arthropoda^ even 

 those of the alimentary canal, are striated. Those of the 

 body and limbs are often attached by chitinized tendons to 

 the parts which they have to move. As the hard skeleton is 

 hollow and the muscles are inside it, it follows that the bodj', 

 or a limb, is bent toward that side of its axis which is oppo- 

 site to that on w^hich a contracting muscle is situated. 



Sounds are produced by many Insects ; but in most cases 

 they cannot be properly referred to a voice, in the sense in 

 which that term is applied to the sounds produced in the 

 higher animals, by the vibrations of the atmosphere arising 

 from the impact of a current of air upon the free edges of 

 membranes bounding the aperture of exit of the current. 

 The chirping and humming of Insects often arise from the 

 friction of their hard parts against one another, or from the 

 rapid vibration of their wings : in some instances, however, 

 recent investigations render it probable that they are pro- 

 duced by the action of expiratory currents on tense mem- 

 branes which bound the stigmata. 



Agamogenesis is very common among some groups of the 

 Arthropoda^ such as the Crustacea and the Insecta, but has 

 not yet been observed in the Myriapoda or the Arachnida. 

 It may be effected in one of two ways : 



1. Either individuals which are, by their structure, inca- 

 pable of being impregnated and are therefore physiologically 

 sexless, though it may happen that they more or less approxi- 

 mate females morphologically, give rise to offspring ( Cecido- 

 myia larvae, Aphis) ; 



2. Or individuals which are capable of being impregnated, 

 and are thus both morphologically and physiologically true 

 females, give rise to eggs which develop without impreg- 

 nation. (The queen-bee, so far as the production of drones is 

 concerned ; many lepidoptera). 



The cases of Apus, I)aphnia, and Cypris, would belong 

 to the Litter category, if it were certain tfiat the very same 

 females which, for a certain period, produce young agamo- 

 15 



