By Arthur V. Evans
My first forays into the field after a long, cold winter often produce photographic opportunities that are all too often missed or ignored when the arthropod photo season really heats up later in the year. One such opportunity presented itself in early March in the form of some tiny arthropods known as symphylans.
Resembling very small (2.0-8.0 mm), pale, relatively plump centipedes, symphylans present macro photographers with plenty of challenges. Their distinctly heart-shaped heads bear one pair of antennae, but are wholly lacking eyes. Depending on the species and stage of development, the backs of their 14-segmented body trunks are covered with anywhere from 15 to 24 soft plates.
The first 12 body segments each have two legs armed at their bases with a short, stiff spine and a special sac. The spines probably enable symphylans to maneuver through the soil, while the sacs probably help to regulate their internal salt and water levels. The next-to-last body segment has a pair of projections from which they produce silk. The last body segment has a pair of long, sensitive hair-like structures.
Brusca and Brusca’s second edition of Invertebrates (2003) places symphylans in the Subclass Symphyla. This name comes from the Greek word symphyo, meaning, “glued together.” Fossils symphylans range in age between 24 million and 64 million years old, but the group is believed to be much older. The nearest relatives of symphylans are members of the Class Myriapoda that includes the Subclasses Pauropoda (pauropods), Diplopoda (millipedes), and Chilopoda (centipedes).
Symphylans are found on all continents except Antarctica. There are about 200 species of symphylans worldwide, but the fauna of North America is so poorly known that a meaningful number of species is not readily available. They occur in both natural and agricultural habitats and are usually found in the upper one-meter of moist, but not wet, soil. Symphylans are usually found in large numbers and sometimes gather in groups. They frequently move up and down in the soil to maintain the proper moisture levels in their surroundings. All known symphylans eat mainly roots and fungal hyphae, and many species are suspected to be omnivorous.
Little is known about symphylan reproductive behavior, other than they reproduce sexually. But this doesn’t mean they actually get together in order to mix their sperm and eggs. For example, a single male garden symphylan (Scutigerella immaculata) produces up to 450 sperm packets in his lifetime and places each and every one of them one the tip of a short, erect stalk of silk. Female garden symphylans amble through these patches of sperm packets, gobbling up to 18 of them a day. While most of the packets are swallowed and digested, some are tucked away in special sacs just inside her mouth. Later, she gently removes eggs from her reproductive organs and fertilizes them in her mouth.
Once fertilized, the female will deposit up to 25 pearly-white eggs in a single mass. At first, the hatchlings are very inactive, have fewer body segments than adults, and only six or seven pairs of legs. As they molt, or shed their exoskeletons, the young garden symphylan adds an additional body segment and pair of legs until they reach adulthood with the full compliment of body segments and legs.
Late winter and early spring is a good time to search for these and other fascinating arthropods that are normally overlooked or ignored in the presence of their larger and flashier brethren. Happy hunting!
©2010, A.V. Evans