Insects and Spiders - What's The Difference?

This quarter, I’m taking an excitingly titled course called “The Public and Private Lives of Insects.” The focus of the course is on the interactions between humans and insects (how they shape our economy and agricultural practices), but to begin, we discussed what exactly an insect is. I had always thought that insects were all of the tiny creatures that one finds crawling in the dirt, climbing up trees, or buzzing around your picnic table, but it turns out that the scientific classification of what constitutes a member of the class Insecta is very specific.

Within the Phylum Arthropoda, along with true insects, are also the myriapods, which include centipedes, millipedes, and other many (myria-) segmented creatures, the arachnids, also known as spiders, and other crustaceans, like lobsters and shrimp. The defining characteristic of all of these arthropods are their segmented bodies. If you look closely at any of the creatures I just mentioned, you’ll noticed that their bodies consist of rigid, exoskeletal segments, the most of which have appendages attached to them.

What characteristics define an insect in particular then? To be classified as an insect, a creature must consist of three body segments: the head, the thorax, and the abdomen. The creature must also have 3 pairs of legs, 2 compound eyes, and usually two pairs of wings. Spiders, in contrast, only have 2 body segments, 4 pairs of legs, and 3-4 simple eyes. Belonging to the class Insecta are creatures such as flies, wasps, ants, butterflies, and beetles.

So next time you want to call something a “disgusting insect!” and crush it beneath your boot, make sure to stoop down and count its legs and segments first, to make sure that you aren’t actually about to crush an arachnid or myriapod first.

Bonus: If you want to watch to watch some intra-phylal warfare, check out this video of a scorpion fighting a wasp.