Arthropods of Edgewood

By Paul Heiple

The first Arthropods that exited the ocean came on to land along with the first plants. They were not insects, but members of the Chelicerata subphylum that includes arachnids. These are all the creepy crawlers such as scorpions, spiders, and ticks. Perhaps it is their very ancient origin that turns us off to them, but they are very important and common arthropods, many of which are found in Edgewood.

The Chelicerates

The features that set the Chelicerates apart from all the other Arthropods is the lack of antennae in all stages of their lives. Their bodies are divided into two sections, one the head and thorax combined (the prosoma) and the other an abdomen (the opisthosoma). The first pair of appendages, the chelicerae, are modified into feeding structures and the second pair are the pedipalps, which perform vastly different functions in the different orders. The remaining four pairs of appendages are considered legs but are also modified in some orders to perform sensory functions. The abdomen also had four pairs of appendages but these were modified to gills in the earliest forms and are now internal and used in respiration.

The Arachnida class of Chelicerates

The largest class of Chelicerates is the Arachnida. This class is the only class in the subphylum found on land and therefore in Edgewood. The most familiar of the Arachnida are the order Araneae, the spiders. This is not the largest order in the class in terms of species numbers; it is the second largest after the Acarina, the mites. But many of the species are large and conspicuous animals, unlike the mites which can often be very small to microscopic. In the spiders, the chelicerae consist of a fang and a basal segment with a grave into which the fang can fold for storage. The pedipalp of the female is leg-like and is used to taste food. The male spider’s pedipalp is modified and used in mating. This makes the sex of spiders easy to  determine; the pedipalps of males have a bulbous end segment. The production of silk is the other feature that seems to set spiders apart from the other orders, but this feature is not unique to the spiders, it is just taken to such a great extent because of the size of the silk structures and the wide range of uses the spider have for silk. The silk is produced from structures at the posterior end of the spider called the spinnerets. The only member of order Acarina you are likely to notice without effort is the ticks. Ticks are the largest mites.

Deer ticks
Deer ticks next to a dime

They are entirely parasitic and therefore not typical of all mites. Features they exhibit are chelicerae modified for puncturing skin and hooks for anchoring the animal in the skin while they feed. The chelicerae are obscured when the animal is not feeding by the leg-like pedipalps. By far, most mites in Edgewood are found in the leaf litter or as small parasites on animals and plants. We are unaware of their presence yet they are important parts of the ecosystem.

A third order of arachnids that I have observed in Edgewood is the Opiliones or harvestmen, also known as daddy longlegs.

Harvestman Microcina sp.
Harvestman (Microcina sp.)

This is a small order compared with the two previous orders with less than a tenth the number of species worldwide. Many people think they are spiders but they have some major differences. The chelicerae are small and lack fangs; these animals do not have venom. The pedipalps are similar to those of spiders. The second pair of legs are often elongated and serve the function of antennae. The body is not divided into two parts and segmentation of the body is always visible. They also produce no silk.

Besides Araneae, Acarina, and Opiliones, three other orders are very likely to be in Edgewood, but I have yet to find them. They are the Scorpiones, the Pseudoscorpiones and the Solifugae. Scorpions


have been collected from Edgewood; they are familiar to us from the form of the pedipalps which are modified into pincers. The tail is actually the last seven segments of the abdomen ending in a stinging barb. Pseudoscopions are small and lack the tail.


Solifugae are known as sun spiders. The have huge chelicerae, often larger than the head of the animal. The pedipalps are long and leg like, giving the animal a ten-legged look.

Sun spider
Sun spider

They have an adhesive organ at the ends used to capture and hold prey. The first pair of legs is reduced and used as tactile organs. The animal then uses six legs for locomotion like insects. It would be nice to have sharp-eyed docents look for these animals and report them in the next few years.


In the park now, look for the lacewings. Members of the order Neuroptera of the class Insecta (not arachnids), these insects are about two centimeters long, the body thin and either


green or brown with finely veined large transparent wings held-tent like over the body. The head is large with large eyes which are golden or copper in color and bearing long antennae. The green lacewings tend to inhabit 

the open areas and the brown lacewings in the wooded areas. Both the adults and larvae feed on small soft-bodied insects such as aphids. The larvae have the look of small alligators with large tong-like jaws. The eggs are held up from the stems and leaves they are attached to by long stalks. These insects are not only beautiful, they are important controls on aphids.


Picture Credits

This article was originally published in the Edgewood Explorer, March 2007.

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