They're not bugs, darnit! (or: what to call that creepy-crawly thing on the wall)

Back in August Angry Professor linked to What's That Bug?, a site featuring photographs and basic descriptions of a huge number of terrestrial invertebrate animals (insects, spiders, etc.) The site's primary goal is to help people identify unknown invertebrates, and they do an excellent job of it.

However, there's one problem: the site uses the word "bug" to describe everything on their site. When I originally wrote this post (back in August), their homepage used the word "bug" at least 67 times, yet only three of the organisms pictured on their homepage were actually bugs.

The problem with the word bug is that while in colloquial English it is typically used to mean something like "any creepy-crawly invertebrate animal I don't like", entomologically speaking the word bug refers only to a specific sub-group of insects, hemipterans.

To understand what hemipterans (true bugs) are, let's review some animal taxonomy. First, animals can be differentiated into a number of phyla (of which the following are just a few):

• Phylum Cnidaria - sea anemones, jellies, etc.
  
• Phylum Annelida - segmented worms, e.g. leeches, earthworms
  
• Phylum Arthropoda - animals with a hardened exoskeleton and jointed appendages, e.g. crustaceans, spiders, insects
  
• Phylum Echinodermata - sea stars, sea urchins, etc.
  
• Phylum Chordata - tunicates, mammals, reptiles, amphibians, etc.

Only organisms in phylum Chordata, subphylum Vertebrata are vertebrates, and thus a correct term for any other animal is "invertebrate". So, if all you know about an animal is that it is not a vertebrate, use the term invertebrate to describe the animal and you can't be wrong.

Invertebrates are vastly more diverse than vertebrates: there are more than 30 invertebrate phyla of animals, and more than a million named species. However, one lineage of invertebrates contains most of their terrestrial diversity: phylum Arthropoda (arthropods). This one phylum contains a majority of the known species of organisms in the world (see the end of this post for more information), and can be broken up taxonomically into four major subgroups on the basis of anatomical differences:

• Chelicerates - spiders, scorpions, etc.
  
• Myriapods - centipedes and millipedes
  
• Crustaceans - barnacles, isopods (e.g., pill bugs), decapod crustaceans (e.g., lobster, shrimp, crab ... all the tasty ones), water fleas (e.g., Daphnia)
  
• Hexapods - primarily including insects, such as grasshoppers, butterflies, flies, bees and wasps, ants, bugs, etc.

So, we clearly must be careful when using the term insect: an insect is an organism in a specific subgroup (class Insecta) of the phylum Arthropoda. Spiders, centipedes, isopods, shrimp, and all the other non-insect invertebrates are not insects, and thus should never be called insects.

Insects are incredibly diverse (with more than 750,000 named species), and can be further broken up into a number of distinct lineages (taxonomic orders). Here are a few of the orders in class Insecta:

• Order Lepidoptera - butterflies and moths
  
• Order Hymenoptera - bees, wasps, and ants
  
• Order Orthoptera - grasshoppers and crickets
  
• Order Coleoptera - beetles (the most diverse group of insects)
  
• Order Diptera - flies
  
• Order Isoptera - termites
  
• Order Siphonaptera - fleas
  
• Order Odonata - dragonflies and damselflies
  
• Order Mantodea - mantids (praying mantises)
  
• Order Hemiptera - true bugs

Only hemipterans can technically be called bugs; all the rest of the insects are simply insects (or, more generally, arthropods or invertebrates). Thus, since the What's That Bug website features spiders, scorpions, and isopods, in addition to insects and hemipterans, they should really be using the term invertebrate or arthropod, not bug. Sadly, "What's That Invertebrate?" just doesn't have the same ring to it, I guess.

But what are hemipterans? Hemipterans all have piercing mouthparts that they use to feed on either animal or plant fluids, and some can transmit human diseases (e.g., Chagas disease). Hemipterans also generally exhibit hemimetabolous metamorphosis, meaning that they do not have a distinct larval stage like caterpillars/butterflies; instead, juvenile hemipterans look just like small versions of adult hemipterans (as do juvenile grasshoppers and crickets).

Hemipterans can be grouped into two suborders: Heteroptera and Homoptera. The group of heteropterans people are most likely to know are water striders, though the suborder does contain many other lineages, such as toad bugs, creeping water bugs, giant water bugs, waterscorpions, backswimmers, damsel bugs, minute pirate bugs (most certainly Pharyngula's favorite insects), lace bugs, stink bugs, assassin bugs, ambush bugs, and yes, even bed bugs.

Readers are probably more familiar with homopterans, as that suborder includes cicadas, aphids, whiteflies, leafhoppers, scale insects, and spittlebugs.

So, next time you see an invertebrate flying around the house, think twice before screaming out, "Oh no! It's a bug!"

Bland, Roger G. 1978. How to know the insects. McGraw-Hill.