NO. 3 



INSECT HEAD SNODGKASS 



57 



The eye stalks of the decapod crustaceans arise from the ends of a 

 transverse ridge on the top of the protocephalon, and project later- 

 ally from mider the base of the rostrum, the latter being a process of 

 the anterior edge of the carapace, and, therefore, from the tergum 

 of the mandibular segment. Each eye stalk (fig. 17 B) consists of two 

 movable segments, a narrow basal one forming a short peduncle, and 

 a large terminal one capped by the hemispherical compound eye. 

 Schmidt (1915) enumerates ten individual muscles for each eye stalk 

 in the crayfish, the basal segment being provided with muscles arising 

 on the head walls that move the appendage as a whole, while muscles 

 from the basal segment move the terminal eye-bearing segment. The 

 eye muscles are innervated by an oculo-motor nerve arising from the 

 brain near the base of the sensory optic nerve. 



THE ANTENNAE 



The insect antenna is typically a many-jointed filament. Usually 

 the first two basal segments are dififerentiated from the rest of the 



B 



Fig. 23. — The antenna. 



A, diagram of typical segmentation and articulation of an insect antenna. 

 B, head of a chilopod, Scutigcra forceps, dorsal, showing dorsal articulation of 

 antennae, and origin of antennal muscles on walls of cranium. 



Ant, antenna; as, antennal suture; E, eye; 11. articular pivot of antenna; 

 Pdc, pedicel ; Sep, scape ; FI, flagellum. 



shaft (fig. 23 A). The first segment serves to attach the antenna to 

 the head, and, being often, thicker and longer than the others, forms a 

 basal stalk, or scape (Sep), of the appendage. The second segment, 

 or pedieel (Pde), is short, and in nearly all insects contains a special 

 sensory apparatus known as the organ of Johnston. The part of the 

 antenna beyond the pedicel is termed the flagelliiiii or clavola (Fl). 

 The flagellum may be long and tapering and made up of many small 

 segments, or it may be abbreviated, and reduced even to a single seg- 

 ment. The scape is set upon a small membranous area of the head 

 wall, sometimes depressed to form a cavity, or autcnual socket. 



