How Webs Work
Spiders produce different types of silk for different purposes, including draglines, egg sacs, ballooning, building a web and wrapping prey. Much of the silk spiders use to spin webs has a sticky consistency, in order to catch flying insects. It turns out that sticky silk isn’t the only reason spider webs are such efficient insect catchers.
According to scientists at Oxford University, not only is much of a spider’s web silk sticky, but it is coated with a glue that is electrically conductive. This glue causes spider webs to reach out and grab all charged particles that fly into it, from pollen to grasshoppers. Physics accounts for the web moving toward all airborne objects, whether they are positively or negatively charged.
According to Prof. Vollrath of Oxford University, electrical attraction also drags airborne pollutants (aerosols, pesticides, etc.) to the web. For this reason, it’s been suggested that webs could be a valuable resource for environmental monitoring. (Thanks to Elizabeth Walker and Linda Fuerst for introducing me to this phenomenon.)
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Caddisflies Laying Eggs
Most caddisflies lay their eggs in or near ponds or streams. A very few species (in the family of northern case makers, Limnephilidae) deposit their eggs above the water on aquatic vegetation in a one- to-two-inch-long mass of jelly (some species’ eggs lack the jelly). Up to 800 eggs (the tan spots within the jelly in yesterday’s post) are laid at one time in one mass. Depending on the species, the eggs take from several weeks up to ten months to hatch. These masses are usually situated so that once the eggs hatch, the larvae will drop down into the water, where they will spend their larval and pupal stages.
Caddisflies are closely related to butterflies and moths, and one of the features they have in common is that the larvae have silk glands in their lower lip. Thanks to the ability to spin silk, the caddisfly larvae build portable cases or attached retreats out of natural material that is available. Some species build elongate tubes out of pieces of plants, sand, sticks or pebbles and reside in them while they drag them along with them wherever they go. Other species attach their cases with silk to crevices in or the bottom of stones in streams. Each species of caddisfly larva always constructs the same type of case, so that you can often tell the genus or even species of caddisfly by the appearance of its case.
The larval stage of a caddisfly can last two to three months or up to two years, depending on the species. Most species spend the winter as active larvae. When it is ready to pupate, the larva attaches its case with silk to something immoveable, such as a large rock. Inside its case, the larva spins a cocoon and eventually pupates inside of it. In two to three weeks the sharp-jawed pupa cuts its way out of its cocoon and floats up to the surface of the water where it emerges as a winged adult, often using its pupal skin as a raft for support during this process. Adult caddisflies live for about 30 days, during which time the males form mating swarms to attract females. After mating takes place, the egg-laying begins.
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Sac Spider Shelter Serves as Nursery & Coffin
It 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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Fern Balls
At this time of year, many new sterile fern fronds have “fern balls” at their tips – something has taken the last few inches of the tip of the frond and stitched it together into a ball-shaped shelter bound with silk. If you open one of these balls, you may find frass – droppings from the immature insect that was dwelling within the ball while consuming the terminal leaflets of the fern. Sometimes, but not always, you’ll find the larva responsible for the frass. Many species of ferns, as well as other plants, are host to many species of larvae, and many of these larvae are immature moths. Pictured is Christmas Fern, Polystichum acrostichoides, which is likely the host of the larva of Herpetograma sphingealis, the Serpentine Webworm Moth, or its close relative, H. aeglealis. Larvae live in these shelters for about a month before pupating and emerging as small, brown moths.
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Jumping Spider Guards Egg Sac
Spiders protect their eggs by wrapping them up in a sac they make out of silk. Some species (such as garden, or black-and-yellow argiope, spiders) then die, leaving their egg sac to withstand the elements, as well as potential parasites and predators, on their own. As you would guess, these sacs are usually fairly impenetrable. In other species, female spiders survive long enough to guard their eggs until they hatch, or even until the young spiderlings disperse, and these sacs are usually far less tough. In the species where the female protects her eggs, some females carry their egg sacs with them at all times (wolf spiders, nursery web spiders) while others (jumping spiders) simply remain with the sac. Their excellent eyesight and impressive ability to leap many times their body length gives jumping spiders an advantage over any potential predators. (Photo – jumping spider with egg sac)
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