The first antennae of arthropods are divisible into two main types, viz. (1) segmented antennae and (2) annulated antennae. In the first type the antenna consists of a variable number of segments, each having intrinsic musculature. The antennae of the Ohilopoda, Diplopoda, Pauropoda, Symphyla, Collembola, and Thysanura Entognatha all pertain to this type; also, the first antennae of many Copepoda and Ostracoda.
In the second type the antenna consists of a peduncle or protopodite composed of one or more segments, each with intrinsic musculature. Distally it bears an annulated flagellum, or a pair of flagella, devoid of intrinsic muscles and whose movements are effected by muscles originating within the peduncle. The first antennae of the Crustacea Malacostraca and the antennae of the Thysanura Ectognatha and of all the Insecta Pterygota belong to this type.
The greatly elongated antennae of the Schizotarsia are intermediate between these two types. They are composed of an immense number of small annuli and an elaborate intrinsic musculature is present, thus allowing these appendages a wide range and flexibility of movement.
The absence of intrinsic muscles in the antennal flagellum throughout the Insecta Pterygota and the Malacostraca appears to be irreversible in the sense that such muscles are never reacquired. It thus lends support to the generalization known as Dollo's Law.
The presence of segmented antennae in the Thysanura Entognatha affords additional evidence in support of the Symphylan theory of the ancestry of insects.