NO. 2 ENCYCLOPEDIA OF INSECT ANATOMY — SNODGRASS 27 



Epicranial suture: See Ecdysial Cleavage Line of Head. 



Ecdysial cleavage line of head: This is the familiar inverted 

 Y-shaped line on the front of the head of young insects. The stem 

 of this line on the top of the head is continuous with the ecdysial 

 line on the back of the thorax. The arms of the cleavage line ordi- 

 narily diverge downward on the face at various angles, but in some 

 hymenopterous larvae the stem is unbranched and continues straight 

 down the face to the labrum. Though the cleavage line on the head 

 has long been known as the "epicranial suture," and regarded as an 

 important structural feature of the cranium, it is in no sense a 

 "suture." It is merely a preestablished line of weakness where the 

 head cuticle will split at ecdysis. A remnant of the line is rarely 

 retained as a groove on the adult head (Snodgrass, 1947). 



Antenna: The antennae are paired segmented appendages of the 

 head of the trilobites and most of the mandibulate arthropods. They 

 are absent only in the Protura, the chelicerates, Limulus, the 

 arachnids, and some insect larvae. In the diplopods, chilopods, 

 symphylans, and the entognathous hexapods, the antennae consist 

 of a variable number of divisions each of which is provided with 

 muscles inserted on its base and arising in the proximal divi- 

 sion. Such antennae, therefore, are fully segmented, and probably 

 represent the primitive antennal structure. In the Thysanura and the 

 pterygote insects, however, the antenna consists of a basal stalk or 

 scape, a small intermediate pedicel, and a distal flagelhim which is 

 usually subdivided into a variable number of annuli. The only 

 muscles in an antenna of this type arise on the scape and are in- 

 serted on the base of the pedicel. The flagellar annuli have no 

 muscles and vary in number from one to many ; evidently the flagel- 

 lum is a single subdivided segment. The pedicel contains a large 

 sensory organ known as the Organ of Johnson. If the pedicel is a 

 separate segment then it must have lost its muscles. The thysanuran- 

 pterygote antenna consists of not more than three segments. 



Each antenna is movable as a whole by muscles arising in the head 

 and inserted on its base. These muscles usually arise on the anterior 

 arms of the tentorium. The whole antenna is set into a membranous 

 socket of the head wall, and is pivoted on a point on the lower rim 

 of this socket. Thereby it is freely movable in all directions. 



The antennae serve as delicate organs of touch and are the prin- 

 cipal seat of the olfactory sense of insects. In addition, some in- 

 sects, such as the male mosquito, hear at least the tone of the female's 

 wing vibrations by means of sensory hairs on the antennae. 



