Smaller Orb Weavers: Similar, But Not The Same
With all projects and efforts, it is good to occasionally step back to revisit the purposes and end goals of what you’re doing. At Harmony Hill, we see these “Nature Notes” as a vehicle to ignite curiosity, to show the wonders of nature and to help teach identification techniques to our fellow naturalists. From time-to-time, we like to spotlight species that may not be anyone’s favorite (our discussions on snakes are a prime example); but, have a unique beauty and wonderful purpose on the landscape. A couple weeks ago, we talked about late summer spiders that are showing up at Harmony Hill. The spiders in that post are harmless to humans, yet are still feared by people. We hope the post opened a few eyes to the purpose these amazing architects serve as pest control in the forest. This week, we hope to shine a light on a couple more species that are often mistakenly identified and who both help keep insect populations in check as the late summer days give way to early fall.
As we’ve previously discussed, the most obvious spiders this time of year are the orb weavers. For clear reasons, their ornate, beautifully functional, and ingenuously built structures draw our attention to them as we walk from fields to forests. Every now and then, our attention is drawn in as we get a face full of their spun silk! Even we find ourselves flailing and pulling web from our face and head, as though we are fighting some invisible enemy who attacked us. Let’s take another look at who might be building some of those webs, how to identify them and see how they’re helping us… even if we don’t realize it as we jitterbug and swing around with spiderweb all over our face.
Sometimes, we get asked about “crab spiders in big round webs in the woods”. We know immediately it is likely one of two spiders, even though neither are actually crab spiders. However, due to their appearance, we know why some make the distinction that they might be. True Crab Spiders belong to the Thomisidae and do not build webs. They are found hiding in flowers and on plants, giving them another common name – Flower Spiders. When we hear “crab spiders in big round webs in the woods”, we can take the clue of web and exclude Thomisidae spiders. With that same clue, we can instead focus our attention on the two spiders we’ll look at in this post!
Looking very much like a crab, the Black-and-White Spiny-backed Orbweaver (Gasteracantha cancriformis) can be found hanging out in its orb-shaped web during daylight hours. The hardened exoskeleton has spiny protrusions around the abdomen, giving it a fierce look. That hardened exoskeleton is a great protection from predation, as it can withstand a great deal of pressure for an animal so small. And the spines are capable of piercing soft tissues, like gums of predators and thinner-skinned areas of a potential predator’s body. These spiny-backed spiders come in a variety of color morphs (polychromatic), from black and white, to black and yellow, and even black and white, with red!
The resemblance to crabs isn’t simply limited to appearance, as the scientific name even alludes to the similarity. The genus name of Gasteracantha refers to spiny (acantha) stomach (gaster), while the species name of cancriformis means “taking the form of a crab”. Cancri is a direct reference to the crab constellation, Cancer, while formis means “in the form of”.
When you find an orb web in your walks in nature, take a look around the edges of the web. Do you see more pronounced and thicker white dashes all around the perimeter? If you do, you may very well be dealing with this species, as members of the spiny-backed family of spider place morse code-looking dashes and dots along the exterior of their orbs. You might wonder why they do such a thing. Well, the prevailing hypotheses are that the dashed allow the web to be more readily seen by birds and, therefore, won’t be destroyed by something far too big to capture flying through, and that the dashes may steer insects more towards the center of the web. As is often the case in nature, and as we’ve talked about in the past, it is likely more than one reason and is a combination of those.
Looking somewhat similar to our first species, yet a member of an entirely different genus, we have another crab-like spider. The Arrowhead Orb Weaver (Verrucosa arenata) is another species we consider when given that “crab spider n a big round web” description. The common name is an obvious nod to the arrowhead-shaped abdomen, with the genus name of Verrucosa being a reference to it looking like a wart. The translation of the species name could be a nod to it having an overall sand color (arena means sand), or it could also bear reference to it being a spider (Araneus means spider and spiderweb).
Continuing the complication of similarities to our first species, this spider is also polychromatic! The females have abdomens that might be white, yellow, or pink! Unlike (Gasteracantha cacriformis), this spider does NOT have spines. And, more importantly, the Arrowhead Orb Weaver sits with its head facing upwards almost always as it rests in the web. The other orb weaving species of spiders in our area sit in their webs with their heads facing down and their abdomens facing upwards!
Finally, let’s talk about how important these spiders are to pest control. They are considerable smaller than the orb weavers we talked about a couple weeks ago and their webs are smaller. With those smaller webs, the radial lines are closer together and capable of capturing much smaller prey. If we told you these spiders are important for controlling mosquitoes and other biting insects, would you begin to see how critical they are at keeping pest populations in check? We kind of thought you would. And it is true! These two species do capture and eat considerable numbers of mosquitoes and small biting insects, making them great friends to have in the forest. (Even if we do walk face first into their webs!)