Kelly’s Field Notes
Let’s talk about Taxonomy!
Taxonomy is how we classify life in biology. Every organism on this planet comes from a common ancestor and taxonomy allows us a map of that relatedness. To simplify this, let's use insects as an example. Insects belong to Kingdom Animalia, Phylum Arthropoda, Class Insecta. Within class Insecta are 28 (extant/living) Orders, broken down into Families, then Genera, and finally Species. Think of this as a branching tree, with Kingdom at the bottom. As you move away from Kingdom, organisms are grouped closer and closer together based on relatedness. Using our friend Isabella Tiger Moth as an example, Isabella belongs to Genus Pyrrharctia of which contains two Species (Pyrrharctia isabella and Pyrrharctia genini). These two species are much more closely related than Genus Pyrrharctia is to other many many genera within the Family they belong to, Erebidae.
In taxonomy and evolutionary biology we love to group organisms by common ancestor, referring to the term paraphyletic. For a non-bug example, think of birds grouped with reptiles because they are descended from coelurosaurian theropods. As an insect example, bees, wasps, ants, and sawflies all share a common ancestor and belong to Order Hymenoptera.
What are we calling a bug?
Colloquially, any terrestrial arthropod can be referred to as a “bug.” Within the context of the podcast, we are using the term “bug” this way, to describe any organism within Insecta, Arachnida, Myriapoda (millipedes, centipedes, and their relatives), and Entognatha (springtails and their relatives). Could we include crustaceans in this and other arthropods? Maybe! For some episodes we might branch out. I don’t see why we wouldn’t call amphipods bugs (they are crustaceans), especially the terrestrial species. My father also calls shrimp sea bugs, for example.
What is an arthropod?
All arthropods have 6 main characteristics:
They are invertebrates.
They have an exoskeleton made of chiton.
Their bodies are segmented (other than mites, which have lost their segmentation).
Their limbs have joints.
They have an open circulatory system.
They are bilaterally symmetrical (if you draw a line down the middle of their bodies each side is the same).
What are the different types of arthropods?
There are around 10 million species of arthropod, so listing all the different types is not going to happen here, but typically Phylum arthropoda can be divided into 4 subphyla:
Artiopods - an extinct group of marine animals (trilobites!)
Chelicerates - named for their mouthparts (chelicerae) or in some cases their tiny claws (scorpions, horseshoe crabs). This group includes terrestrial arachnids, horseshoe crabs, and sea spiders.
Myriapods - this group is all terrestrial and has many segments, each continuing one or two limbs. Centipedes, millipedes, pseud-centipedes, and pauropods. (tiny millipede-like animals).
Pan-Crustaceans - Animals in this group are mostly aquatic (other than terrestrial isopods and hexapods) and have biramous appendages (appendages that branch into two). This is a huge subphylum! It encompasses hexapods (insects are in there), barnacles, crustaceans, copepods, branchipods, ostracods, etc. It’s a very long list.
There is some dispute about where trilobites go, but taxonomy is kind of a messy field. A more simplified list is: Chelicerates, Myriapods, Crustaceans, Heaxapods.
Silverfish are small, wingless, nocturnal insects. They are 13-25 mm (0.5-1 in) in length, with a tapered abdomen (like a fish). They are sort of carrot-shaped. As their name implies, they are also silver in color. They are fairly long lived for an insect, living up to three years. You may have seen them in your basement, bathroom, or attic as they prefer damp, dark spaces.
While their body plan may be difficult to distinguish, they are built like any other insect with a head, thorax, and abdomen. Their mouthparts are for chewing, as they feed on a wide variety of materials - plants, dead insects, glue, flour, and oats. They are a huge pest if you have a lot of books improperly stored in your basement or attic. This is due to the cellulase they have in their midguts, which allows them to break down cellulose.
Silverfish go through three stages: egg, nymph, adult. Silverfish undergo a type of metamorphosis called ametabolous development (simple metamorphosis), which means they go through very few changes from nymph to adult. This is in contrast with say grasshoppers, who go through paurometabolous development (gradual metamorphosis), which is more gradual and requires more physical changes. Insects also undergo hemimetabolous development (incomplete metamorphosis), where adults lose some traits nymphs had (think dragonflies) or holometabolous development (complete metamorphosis) where larvae do not resemble adults at all and must go through the pupal stage (think butterflies).
A female silverfish will lay less than 100 eggs in her lifetime, which take two weeks to three months to hatch. A really fascinating thing about silverfish is their molting. Generally, when we talk about insects molting they go through maybe 4 or 5 molts until they become adults. While it may take a nymphal silverfish 6 to 7 molts to become an adult, they will molt a total of between 17 and 66 times, continuing to molt as adults! Some have been recorded molting 30 times in one year. The time it takes them to go from egg to adult could take three months, or their entire three year lifespan.
Speaking of eggs, many insects, such as our friends the silverfish, have courting rituals. Male and female silverfish spend thirty minutes doing a sort of three part dance! In the first part, they stand face to face, vibrating their antennae over each other. They repeatedly back off and come back together to do this. In the second part, the male makes a break for it while the female chases after him. When the chase has ended, they stand side by side, head to tail, and the male vibrates his tail against the female. Once all this is over the male creates a spermatophore (a pouch containing sperm) which the female places in ehr ovipositor to fertilize her eggs.
Aak, Anders, et al. "Long-tailed silverfish (Ctenolepisma longicaudata)–biology and control. Revised edition-2019." (2019). https://www.fhi.no/globalassets/dokumenterfiler/rapporter/2019/long-tailed-silverfish-ctenolepisma-longicaudata---biology-and-control.pdf
Houseman, Richard M. "Silverfish and firebrats." (2007) https://mospace.umsystem.edu/xmlui/bitstream/handle/10355/51205/g7376-2007.pdf
Kaleka, Amritpal Singh, Navkiran Kaur, and Gaganpreet Kour Bali. "Larval development and molting." Edible Insects 560 (2019): 17.
“Silverfish.” Wikipedia, 13 Nov. 2021, en.wikipedia.org/wiki/Silverfish.
Why do we call some bugs beneficial and others pests?
These terms are very anthropocentric (human-centric) and generally do not relate to the niches a bug is filling in the wild. Beneficial bugs have “jobs” that are good for humans and the things we care about. Lady bugs, for example, eat bugs in our gardens that would otherwise harm our plants (like aphids). Spiders, predatory beetles, predatory mites, mantids, and other predators also fill this niche. Bugs that pollinate are beneficial because they make sure our agriculture prospers, or we have beautiful flowers to place in and around our homes.
Pests are the opposite of beneficial insects. Pests destroy our crops, bite us and our pets, spread disease, and generally cause harm to things we value. Mosquitoes are considered pests despite their role as pollinators because they cause us harm and even death. Aphids are pests that feed on and damage our roses and vegetables. Cockroaches, drain flies, carpet beetles, ants, and other insects you find in your home that damage your food and property or can make you sick, are all pests.
Kelly, you have typed the word “niche” several times now. What does that mean?
Niche is the role an organism plays within its community. In ecology, a community is a group of organisms that interact with each other (plants, animals, fungi, etc.). This role is based on resource consumption, spatial distribution, temporal distribution, interaction with other organisms, etc. A niche can be general or specialized and according to the competitive exclusion principle, no two organisms can occupy the same niche for very long. An organism occupying a specialized niche requires very specific environmental parameters to survive. Some species are both generalists and specialists as they move through their life cycles (monarch caterpillars are specialists feeding on milkweed, but adults are generalists feeding on a variety of plants for nectar).
A special question from Amanda: Do they keep their organs in their booties?
An arthropod’s organs are held in their abdomen (the booty). The head or cephalothorax (combined head and thorax) contain the eyes, cerebral ganglia (like a brain), antennae, and mouthparts. The thorax may contain specialized glands or a crop (depending on taxa) and the aorta, ganglia, and trachea run through it to the abdomen. The abdomen holds everything else; mid-gut, hind-gut, crop (depending on taxa), reproductive organs, hearts, venom, etc.
What organs do bugs have in comparison to humans?
Bugs and humans have more in common than you’d think! 60% of our DNA is the same as a fruit fly’s. We both have eyes, brains, hearts, digestive tracts, and reproductive systems. Where we differ in organs we still overlap in needed functions within the body. For example, insects do not have a liver, but they do remove toxins from their bodies by distributing it through their fat body like a liver. Bugs do not have veins to deliver oxygen, but they do have branching trachea (small tubes) that connect to spiracles (pores open to the environment). The trachea become so tiny as they branch they can deliver oxygen to most of the cells within the body.
How do they breathe? Is there more than one strategy for this?
Most terrestrial bugs breathe through a series of tubes called trachea connected to openings along the sides of their thorax and abdomen called spiracles. A bug can control its spiracles, opening and closing, through muscle contraction. Oxygen enters the spiracles and passively diffuses through the trachea. There are a few exceptions to this setup though. Bug eggs, for example, receive oxygen through small pores called aeropyles. Some taxa are also so tiny they no longer need trachea and instead can breathe through diffusion (springtails are a great example). If we’re going to include terrestrial isopods as bugs, they actually have gills and need to live in a fairly humid environment to pull oxygen into their gills.
If you want to get real weird, let’s talk about parasitoids! Parasitoids live inside of their hosts, but they still require oxygen. There are a few ways they can handle living like this. One way is that they get all the oxygen they need from the host’s hemolymph (blood) they are feeding off of. This is a common method for parasitoid wasps. Parasitoid flies take this a few steps further, by either connecting to their host’s trachea or they use my favorite method, snorkeling. Parasitoid flies lay their eggs on the outside of their host, then start to burrow inside. The host immune system creates a scab around the larva, which the larva eats its way through, this continues as it burrows into the host fat body, all the while the host keeps making scabs (while the larva breaks it). Eventually this becomes a long snorkel like tube giving the larva access to the outside oxygen.
Do all bugs have compound eyes? What are compound eyes really? Spiders have many eyes, do other things have lots of eyes?
Not all bugs have compound eyes. Our friends the centipedes and millipedes (and their relatives) do not have compound eyes. Arachnids and other chelicerates also do not have compound eyes (other than horseshoe crabs) but instead have simple eyes, though some have excellent eyesight and can see color and UV (jumping spiders come to mind). Compound eyes are made up of thousands of small eyes, all containing a cornea, lens and other accessory structures. Each eye sees one image, which is pooled together to see a wider panoramic image. This gives a much wider view with greater capture of polarized light and fast movement detection, but with less image resolution and clarity.
If you want to get really weird, male swallowtails have an eye on their genitals that senses light. Because they mate facing away from females they use this light sensing eye to line up properly. There’s a hindsight is 20/20 joke in here somewhere.
The majority of spiders have eight eyes, but they can have no eyes, 2, 4, 6 or even 12 eyes, depending on species. You can actually narrow a spider down by family by looking at the number and position of a spider’s eyes. Many bugs have multiple eyes, as many have both compound eyes and simple eyes on top of their heads (ocelli). There are other animals that are not bugs that have multiple eyes, such as the four-eyed fish (Genus Anableps) or some reptiles and amphibians have a third eye on their head called a parietal eye or pineal eye, which senses light. Starfish, jellyfish, and bivalves, among other taxa, also have many eyes.
Butterflies taste with their feet? Is this common across bugs?
Many bugs taste with their feet! They have chemoreceptors not only on their feet, but on their antennae, legs, and on their bodies. Tasting with your feet is really beneficial when trying t figure out what plant you’ve landed on or if you’re a cricket, before laying your eggs in soil you’d taste with your ovipositor to make sure that’s the soil type you're looking for. There are chemoreceptors all over on bugs!
What’s up with bug mouthparts? Are there many different types?
Bug mouthparts come in a variety of shapes and sizes, mostly related to how that bug eats. For example, insect mouthparts come in four types: piercing/sucking, chewing, siphoning, and sponging. Chelicerates have fangs or pincers, they cannot chew. Some, like spiders, also have pedipalps which can help hold prey in place. Centipedes, hexapods (including insects), , have three pairs of mouthparts: maxilla 1, maxilla 2, and mandibles. Maxilla are sort of like lips, used to grasp food. Terrestrial isopods have mouthparts made to chew. There is an order of insects called Hemiptera which are often called “True Bugs” and they are characterized by their piercing mouthparts. Aphids, cicadas, and bed bugs are part of this group. But not every insect with a piercing mouth part is a hemipteran; mosquitoes developed their piercing mouthparts independently and are a part of order Diptera..