j^O. 3 INSECT HEAD SNODGRASS 59 



be individually provided with muscles {Scolopendra, Spiroholus). 

 The first antenna, or antennule, of the crayfish, according to Schmidt 

 (1915), has paired antagonistic muscles for each of its first three 

 proximal segments, and the third segment contains a single reductor 

 inserted on the base of the dorsal branch of the flagelluni. but other- 

 wise none of the flagellar segments is provided with muscles. 



The Arachnida and Xiphosura lack antennal appendages in the 

 adult stage. Croneberg (1880) describes a pair of head lobes in the 

 arachnid embryo, which he says fuse into a median rostrum in the 

 mites and in the higher arachnids, and which he believes represent 

 the antennal appendages. Jaworoski (1891) likewise describes in 

 the embryo of a spider, Trochosa singoriensis, a pair of lobes situated 

 before the chelicerae, which he claims are rudiments of the antennae 

 (fig. 22 B, Ant), but he says the lobes disappear during later develop- 

 ment. 



THE POSTANTENNAL APPENDAGES 



The pair of postantennal appendages on the tritocerebral segment 

 of the head, known also as the antennae (Crustacea) , second antennae, 

 premandibular appendages, and intercalary appendages, are at best 

 rudimentary in all insects. According to Uzel (1897), two small 

 lobes in the adult head of Campodea, lying between the labrum and 

 the maxillae, in the space left free by the retracted mandibles, are 

 the tritocerebral appendages ; the writer has found a pair of small 

 papillae in Dissosteira between the bases of the mandibles and the 

 angles of the mouth (fig. 42 B, Put) that might be vestiges of these 

 organs Otherwise tritocerebral appendages are known m msects only 

 as evanescent rudiments in the embryo (fig. 22 D, Put) . In the Myna- 

 poda likewise, the postantennal appendages are lackmg, or possibly 

 are present as temporary premandibular lobes on the head of the 

 embryo ("rudiments of lower lip" in GcophUus, Zograf, 1883). In 

 the Crustacea, on the other hand, the appendages of the tritocerebral 

 segment, though sometimes reduced or lacking, are commonly highly 

 developed, biramous organs, the second antennae, or " the antennae 

 according to the terms of carcinology. In the decapods each appen- 

 dage consists of a two-segmented base (fig. 24 B. Prtp), of a large, 

 one-segmented exopodite (Exp), and of a long, slender endopodite 

 (Endp) of which the terminal segment is the many-jointed flagellum 

 \fI). The exopodite is independently movable by abductor and ad- 

 ductor muscles arising in the second segment of the base 



In Xiphosura and Arachnida. the chelicerae (fig. 24 A) ^^'"^ f "" 

 erally regarded as the appendages of the tritocerebral segment. Their 



