40 SOME ASPECTS OF MORPHOLOGY 



the abductor and adductor muscles. The ventral adductor 

 muscles of the Apterygota either disappear or are represented by 

 a greatly reduced tentorial adductor which is found in the 

 Orthoptera, Isoptera and the nymphs of some of the lower orders. 

 Since the mandibles perform their hardest work by means of 

 inward movement, the adductor muscles are more powerfully 

 developed than the abductors. 



(d) The Antennce. That the insect antenna is the homologue 

 of the crustacean antennule is generally accepted. This con- 

 clusion receives support from the fact that in many of the lower 

 insects the tritocerebrum bear evanescent appendages which are 

 to be regarded as the last traces of the crustacean antennae. It 

 naturally follows that at some remote stage in their evolution the 

 insects and the myriapods, along with them, were derived from 

 some early crustacean form, otherwise the occurrence of these 

 tritocerebral appendages is left unexplained and cannot be 

 accounted for. 



The scape of the insect antenna is the protopodite. and in the 

 majority of insects the muscles which move the flagellum as a 

 whole are located in this segment. In the Entognatha, however, 

 researches by the present writer, not at present published, show 

 that in the Entognatha among Thysanura a very different con- 

 dition prevails. The moniliform antennae of these primitive 

 insects bear muscles in each individual segment and a similar 

 arrangement prevails in the Symphyla and certain other myriapods. 

 The relations of the multiannulate antenna of Machilis and 

 Blatta, for example, with the segmented antenna of Japijx is 

 being studied, but insufficient data are at present available to 

 justify further discussion on the subject for the time being. 



