NO. I THE INSECT HEAD — SNODGRASS 29 



differing in the number of segments they contain. In the ento- 

 gnathous apterygotes (Collembola and Diplura) the antennae vary 

 in length, but are fully segmented and each segment is individually 

 musculated (fig. 14 A). This type of antenna is characteristic also 

 of the chilopods, diplopods, pauropods, symphylans, and some crus- 

 taceans such as the copepods and ostracods. It therefore represents 

 the primitive arthropod antenna. In the Thysanura and Pterygota, 

 on the other hand, there are muscles only in the basal segment (B) 

 insterted on the small second segment (Pdc). The rest of the antenna 

 in these insects is a flagclhim (Fl) of various lengths subdivided into 

 nonmusculated annuli. 



Imms (1939), who first pointed out this difference in the arthropod 

 antennae, distinguished the two kinds as "segmented" and "annu- 

 lated" antennae, but he held that the flagellar annuli are primitive 

 segments which have lost their muscles. The two antennal types have 

 been described also as "musculated" and "nonmusculated," and made 

 a basis for dividing the mandibulate arthropods into Myocerata and 

 Amyocerata (Remington, 1955). However, since the number of 

 annuli in the flagellum varies from one to many, it is evident that the 

 flagellum represents a single segment variously subdivided. This in- 

 terpretation follows also from Imms' (1940) observation that the 

 growth of a fully segmented antenna proceeds by division of the 

 apical segment, while growth of the flagellum results from subdivi- 

 sion of the basal annulus, or sometimes by division of the inter- 

 mediate annuli. The same thing has been noted by other writers. 

 Lhoste (1942), for example, shows that the antennal flagellum of 

 Forficiila increases during growth from 8 to 14 annuli by division of 

 the basal annulus. The thysanuran-pterygote antenna, therefore, has 

 not more than three true segments, the third of which is usually a 

 multiannulate flagellum. The first and second segments are muscu- 

 lated. 



The typical thysanuran and pterygote antenna (fig. 14 B) is a 

 slender elongate appendage composed of three parts, a basal stalk, 

 or scape {Sep), a small middle piece, or pedicel (Pdc), and an annu- 

 lated flagellum (Fl) of variable length. The scape is set on a small 

 membranous area of the head wall with a reinforced margin, and is 

 pivoted, usually from below, on a marginal point, the antennifer (af). 

 The antenna as a whole is thus freely movable in all directions, and 

 is provided with basal muscles inserted on the scape. The antennal 

 muscles in some insect larvae and in other arthropods arise on the 

 head wall, but in most insects they arise on the dorsal arms of the 

 tentorium where these arms make contact with the head wall. 



