An online resource based on the award-winning nature guide

Arachnids

Barn Spiders Spinning

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If there is a fairly large spider spinning orb webs in a corner of your shed or barn and it has striped gray, brown and white legs, chances are great that it is a Barn Spider, Araneus cavaticus. These spiders are nocturnal, so it’s often the early-rising/late-to-bed folks that observe these arachnids. During the day, Barn Spiders hide in a nearby crevice where birds and other predators cannot easily find them.  Webs are freshly constructed every night (or every few nights) and the remains of the old web are eaten in order to conserve the valuable silk. During the night Barn Spiders can be found hanging in the center of their web, awaiting prey.

Male Barn Spiders reach between ¼” and ½” in size and adult females typically are around ¾”. Most males spin webs less frequently and spend much of their lives wandering, attempting to find a female to mate with. Thus, most Barns Spiders you see in webs tend to be females.

The spider in E. B. White’s Charlotte’s Web was based on a Barn Spider. In his inimitable way, White named her Charlotte A. Cavatica, a reference to the Barn Spider’s scientific name. One of Charlotte’s daughters, after asking what her mother’s middle initial was, names herself Aranea.

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Crab Spiders Well Camouflaged

9-8-16-crab-spider-20160822_0529Some say crab spiders derived their common name from the way in which they move sideways like a crab. Others liken their first two (longer) pairs of legs to those of crabs. Still others feel their short, wide, flat bodies resemble those of crabs. Whatever the source of their name, this group of spiders consists of ambush predators. Instead of stalking their prey, or catching them in a silk web, crab spiders tend to stay put (often on flowers), and blend into the background as much as possible in order to pounce on unsuspecting prey.

In order to meet with success, crab spiders camouflage themselves extremely well. Some resemble bird droppings, while others look like fruits, leaves, grass, or flowers. The Goldenrod Crab Spider, a fairly common white or yellow crab spider with pink markings, is capable of changing its color from white to yellow over a period of days, depending on the color of the flower it is on. (Crab spiders often remain in the same location for days and even weeks.) The likeness of the pictured crab spider to one of the Turtlehead’s buds is surely not coincidental.

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Wolf Spiders Closely Guarding Egg Sacs

5-24-16  wolf spider 008Spinnerets, located at the far end of a spider’s abdomen, serve as spigots through which silk is exuded, but they also have another function for some species of arachnids.  Female wolf spiders use their spinnerets to grasp their eggs sacs, enabling them to carry and guard their eggs until they hatch.   In order not to damage the eggs when she moves, the spider tilts her abdomen up slightly.  Catching prey with this added encumbrance and in this position must take great skill.  Once the wolf spider’s eggs hatch, the young climb up on top of the abdomen where they spend their first days before dispersing. (Nursery web spiders also carry their egg sacs with them, but clasp them with their jaws, or chelicerae, and small, leg-like appendages called pedipalps.)

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Sheet Web Weavers Still Active

10-9-15 sheet web IMG_5116Spider webs are constructed in a variety of shapes, for which many of them are named. Among others are orb webs, triangle webs, mesh webs and sheet webs. One of the most prevalent types of spider webs seen this late in the year is the sheet web, made by members of the Lynyphiidae family.

Several different web designs are found in this family including the bowl and doily, dome, and sheet. Tiny sheet web weavers spin small horizontal sheets of webbing and then hang upside-down underneath their web. Some species make two layers and hide between them for protection. When a small insect walks across the web, the spider bites through the silk, grabs its prey, pulls it through the web and eats (actually drinks) it.

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Spiderlings Dispersing

9-25 spiderlings dispersing 615Although many spider eggs hatch in the spring, there are some that hatch in the fall. Most spiderlings stay within the egg sac until they undergo their first molt – their small cast skins can be seen inside the old egg sac. After molting they emerge and cluster together, still living largely upon the remnants of yolk sac in their abdomens. In several days the spiderlings are ready to disperse, which is necessary to avoid competition for food and prevent cannibalism among the hungry siblings.

Some species, especially ground dwellers, disperse by walking, often over relatively short distances. Others, particularly foliage dwellers and many web builders, mainly disperse by ballooning. To balloon, spiderlings crawl to the top of a blade of grass, a twig or a branch, point their abdomens up in the air and release a strand of silk. Air currents catch the silk, often called gossamer, and lift the spider up and carry it off. Aerial dispersal may take a spiderling just a few feet away or much, much farther – spiderlings have been found as far as 990 miles from land. (Charles Darwin noted spiderlings landing on the rigging of the Beagle, 62 miles out to sea).


Spiders Molting Exoskeletons

shed spider skin 052Like other arthropods, spiders have a protective hard exoskeleton that is flexible enough for movement, but can’t expand like human skin. Thus, they have to shed, or molt, this exoskeleton periodically throughout their lives as they grow, and replace it with a new, larger exoskeleton. Molting occurs frequently when a spider is young, and some spiders may continue to molt throughout their life.

At the appropriate time, hormones tell the spider’s body to absorb some of the lower cuticle layer in the exoskeleton and begin secreting cuticle material to form the new exoskeleton. During the time that leads up to the molt (pre-molt period), a new, slightly larger, inner exoskeleton develops and is folded up under the existing exoskeleton. This new soft exoskeleton is separated from the existing one by a thin layer called the endocuticle. During the pre-molt period the spider secretes fluid that contains digestive enzymes between the new inner and old outer exoskeletons. This fluid digests the endocuticle that separates the two exoskeletons, making it easier for them to separate.

Once the endocuticle is completely digested the spider is ready to complete the molt. At this point a spider pumps hemolymph (spider blood) from its abdomen into its cephalothorax in order to split its carapace, or headpiece, open. The spider then slowly pulls itself out of the old exoskeleton through this opening.

Typically, the spider does most of its growing immediately after losing the old exoskeleton, while the new exoskeleton is highly flexible. The new exoskeleton is very soft, and until it hardens, the spider is particularly vulnerable to attack.

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Sac Spider Shelter Serves as Nursery & Coffin

7-10-15  sac spider 018It is not unusual to come across a rolled-up leaf – the larvae of many moths create shelters in this fashion, using silk as their thread. Less common, and more intricate, are the leaf “tents” of sac spiders. With great attention paid to the most minute details, a female sac spider bends a leaf (often a monocot, with parallel veins, as in photo) in two places and seals the edges (that come together perfectly) with silk. She then spins a silk lining for this tent, inside of which she lays her eggs. There she spends the rest of her life, guarding the eggs. She will die before the eggs hatch and her body will serve as her offspring’s first meal. (Thanks to Ginny Barlow for photo op.)

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