Life History & Taxonomic Definitions
An insect is a creature with six legs (“hexapod”) and three body parts, belonging to the taxonomic class “Insecta”. “Six legs and three body parts” is an oversimplification, as it only describes the adult insects and does not account for their “baby stage”, the larvae (or nymphs). Still, it’s a starting point, as it tells us that spiders are not insects (eight legs, two body parts), nor are centipedes, millipedes or rolly pollies (too many legs and body parts). Some insects are not insects at all (as with springtails [class Collembola]), yet share the six-legged framework. All insects are hexapods, but not hexapods are insects, and the Hexapoda clade’s origins predate insect evolution.
I’ve covered much about bees and other hymenopterans, yet I have not defined what a bee is, much less what makes an “insect”: six legs, small and creepy-crawly? Yeah that’ll do. But you want more, don’t you? You want to know more about this lineage of hexapod arthropods who descended from an ancestor shared with crustaceans approximately 480 million years ago. Those “six legs” are a byproduct of their evolution, not their “defining feature”, and we begin by defining an insect:
An Entomologically-Scientific Definition of an Insect:
An insect is an invertebrate belonging to the taxonomic class Insecta. They (generally) lay eggs, go through life in distinct stages (larva-pupa-adult; “holometabolous”) or as smaller adult versions maturing with each iteration (nymph-nymph again-more nymph- yet another nymph-adult; “heterometabolous”). A general adult insect structure consists of six legs and three body segments: head, thorax & abdomen, either with wings or without. Insects can be ecological specialists or generalists, occupying every niche in the biome: deserts, glaciers, mountains, forests, underground, tundras, the nether…
Some insects are specialists (many solitary bees), others are generalists (honey bees, cockroaches), and they (generally) reproduce rapidly – the egg-to-adult stage is measured in days or weeks rather than years. All insects have an important role to play in their niche, and to classify them as “beneficial” or “pests” is a purely human trait. Consider that the same species of hawkmoths (Manduca spp.) are considered a pest in their larval stage (the hornworms that plague my tomatoes), and a pollinator of nightshades in their adult stage (owing to their long probiscis).
Voltinism, or the Number of Generations Produced Per Adult
I could go on and on, so I will: of those rapidly-reproducing insects, many are capable of producing multiple generations (“multivoltine”), with the trend being more common at decreasing latitudes (i.e. closer to the equator). Remember, insects are “exothermic”; the environmental temperatures influence their bodily functions. A warmer climate will lead to more activity, which is why the tropics are ideal for insect life (they also do not have “winters”, just dry and wet seasons).
Not all insects are multivoltine. Even when the environments are favourable, an adult’s lifecycle will produce one or two generations (uni- & bi-voltine respectively). An extreme example are the North American periodical cicadas (Magicicada spp.) who can take 13 or 17 years to reach maturity. A more global (“cosmopolitan”) example are the solitary bees. Adult females build nests in spring and summer, lay eggs and provision them with nectar & pollen, seal the nest and depart. This foundress will be dead by winter, and the new generation will emerge next spring to restart the cycle (unless disease, predators, parasitoids and/or cuckoo bees get to them first).
One school of thought states that the purpose of life is to reproduce, and solitary insects can be viewed in this lens: they exploit the seasonal abundance (spring and summers) to accumulate resources and invest in producing multiple generations1 to spread their bets. If one generation dies due to say, disease or unexpected spring flooding (is there such a thing as “expected” flooding?), then at least the second summertime brood can survive and continue that insect’s lineage.
Why Spiders (Arachnida) are Not Insects (Insecta)
You are forgiven for thinking that spiders are insects, as this is what captures the public eye (and the courts too)… “spiders and other bugs”. So in a lay sense, you are correct to call spiders insects; it’ll keep the conversation flowing and you spare you the uncomfortable glances. Fortunately, you’ve moved past social etiquette and care for this distinction between Arachnida and Insecta. The short answer is in two parts: spider anatomy (eight limbs, two body parts & mouthparts) and phylogeny (evolutionary history) is what distinguishes them from insects.
The Fossil (& Molecular) Record Suggests That Spiders Predate Insects by At Least 18 Million Years
Non-technical readers are welcome to skip this section
“Arthropoda” are the taxonomic phylum that contains insects (subphylum Hexapoda; “six legged”), centipedes & millipedes (previously Myriapoda; “many legged”, now Chilopoda & Diplopoda respectively) and spiders (Arachnida; named after the weaver who in Greek mythology, challenged Athena to a weaving contest, lost and was transformed into a spider after her death). Molecular clock analysis (a technique measuring accumulated genetic mutations) suggests that arachnids came first, “colonising the land near Cambrian–Ordovician boundary (~485 million years ago [mya]), and diversified soon after” (Lozano-Fernandez et al., 2020). By contrast, the fossil record for Arachnida assigns their first appearance to be around 418-423 mya, (Proscorpius osborni, an ancestor of scorpions; Magalhaes et al., 2019).
The oldest fossil record for a hexapod is Rhyniella praecursor (Class Collembola, the springtails), some 400 mya in the Lower Devonian period (Gullan & Cranston, “the Insects”, 5th edition). These early springtails were known by their piercing-and-sucking mouthparts, inflicting damage on fossilised plants from around the same period2. Like spiders, the molecular clock analysis of insect phylogeny suggests aa lineage earlier than 400 mya, however they have yet to identify this lineage. The most “famous” example of prehistoric insects is in the Carboniferous period (~359-299 mya), when oxygen concentrations were far greater than today (~30% vs 20%) and insects became gigantic in response to the increased availability. (I wonder what spiders were doing during this period?)
Insecta-Arachnida Anatomical Differences: Bodyparts and Mouthparts and… Asymmetric Genitalia
This could be an entire article
Distinguishing between bodyparts is easy enough: insects have six legs, spiders have eight. Insects have wings, spiders do not. Insects have an abdomen, thorax & head, whereas spider have a (small) head and a large abdomen. Eyes differ, with eight being the most frequent number in spiders, whereas in insects (like bees & wasps), it is five: two compound eyes and three simple ones (the larval forms also have various eyes, or none at all!). Digestion differs, in that spiders externally digest their food via their mouthpart secretions, whereas insects do ingest and digest largely internally (I wonder if the act of regurgitating and sharing nectar by honey bees is considered “external” digestion).
I could go, but instead I’m leaving you with a (rather-long) summary table at the end of this article. Onto the genitals!
Arthropod Genitalia Provides Valuable Taxonomic Information
Both insects and spiders have gynandrous individuals (those with both male & female reproductive parts), it is the asymmetry of the (largely-male) genitalia in insects and the (largely-female) asymmetries in spiders that are a subject of study. Huber, Sinclair and Schmitt’s review of the topic suggests that “that male insects are much more prone to have genital asymmetries than females. Female asymmetry is not only rare but often inconspicuous, and in at least some cases it evolved after male asymmetry” (Huber, Sinclair & Schmitt, 2007).
It could be that these evolved in response to sexual selection by females, or perhaps such an asymmetry produced different mating positions whereby the male is more secure in his grasp of the female, ensuring reproductive success (see diagram below).
Male spiders carry two independent sperm reservoirs (“pedipalps”), each used to fill the two corresponding female spermathecae. For a full sperm transfer, a male uses both his sperm reservoirs when mating, either both in one go (left & right pedipalps to one spermatheca) or in alternating positions (left pedipalp to left spermatheca, right pedipalp to right spermatheca), making male asymmetry redundant or even unfavourable. This contrasts most winged insects (Infraclass: Neoptera), who “have a single unpaired sperm transfer organ and may thus copulate from one side only and still transfer the full amount of sperm” (Huber, Sinclair & Schmitt, 2007).
In plain terms, it is best for a male spider to mate twice with the same female and empty his sperm reserves to maximise female fertilisation success; a male insect achieves this in a single episode. Yet another reason why insects and spiders are separate entities.
The Evolution & Success of Adult Insects’ Six Legs
The question of how insects developed six legs is just as difficult as the “why” they developed six legs. Looking at the fossil & molecular records, insects diverged from crustaceans3, with the most likely candidate for their sister group being either the Branchiopoda (fairy shrimps, tadpole shrimps and water fleas), the Remipedia (a class of blind crustaceans) or perhaps a fusion of Remipedia & Cephalocardia (horseshoe shrimp), the Xenocardia (“strange shrimp”; Regier et al., 2010).
Taxonomic headaches aside, the ancestral crustacean would have had many legs, that over evolutionary time, led to the emergence of a new species that favoured six legs instead of many. (Those that remained with many legs became the centipedes). So the first insects evolved in the sea during the Cambrian period and later colonised the land as the conditions (i.e. plant life) became suitable. The literature is inconclusive as to why evolution favoured adult insects with six legs rather than four or eight, so I’m going to go out on a limb (pun intended) in an attempt to explain it:
The short answer expands on what was covered above, in that six legs are an evolutionary inheritance from their ancestral hexapods as they successfully compromised between:
- Stability & speed (the more legs you have, the more stable and [theoretically] faster you are).
- Energetic efficiency & neural control (less legs is more energy efficient, as increasing limb numbers require more innervation and musculature for control [again, theoretically]).
- Freeing the forebody (think of it as a primitive head/thorax region) for specialised functions (feeding, sensing, and flight).
It’s not that evolution chose six as the magic number, rather this number of legs evolved (by chance) in the ancestor of insects and since it became highly successful (perhaps for the reasons stated above), it resulted in the growth and dispersal of hexapod populations in the primordial times.
What Can an Insect Do with Six Legs?
Insects with hard exoskeletons can control the contractions of their leg muscles, activating one side and relaxing the other to achieve motion. The six legs are located ventrally (“lower down”) and closer together on the thorax, making it possible to house both walking and flying muscles in that one region.
Slow-to-moderate motions happen in a tripod fashion (elaborated here), whereas fast movements vary according to the species: some increase the frequency of their tripod style, others go from this triangular sequence (a tripod has three legs) into quadrupedy (four legs) or use their hind legs (“bipedality”). Cockroaches are very good at these high speeds, being able to “maintain speeds of up to 1 meter/second whilst twisting and turning up to 25 times per second” (They’re assisted by the antennae, itself one remarkable organ; Gullan & Cranston, “the Insects”, 5th edition).
Some insects developed specialised hindlegs for jumping: crickets, flea beetles and some weevils have an enlarged hind leg femur with large(r) muscles that can produce contractions capable of propelling the insect into the air in a (usually) uncontrolled fashion (Orthopteran enthusiasts can vouch for this). Good for fleeing predators, and also for getting a head start on flight: you’re already in the air, now spread your wings.
The six legs help with swimming, or walking on the water surface, as is the case with waterstriders, who have water-repelling hairs on their legs that maintain the water’s surface tension, allowing them to “stride” or “skate” on its surface by rowing with their legs.
Adult Insect Body Organisation: Head, Thorax & Abdomen
This topic has been previously addressed (rather-poorly) using honey bees as a model, excerpts below
Insects have three body segments: the head, thorax and abdomen. The head contains the eyes, the mouthparts, antennae and brain, the thorax contains the muscles: insect legs, insect wings and their respiratory system. Everything else is in the abdomen – the honey stomach (for bees), the reproductive system and major parts of the circulatory system, as well as their digestive system (and in bees & wasps: the stinger). These features are general and are largely unchanged between insect species. For more on general anatomy, read “The Anatomy of Bees”.
The honey bee’s head is about vision and perception, containing organs for vision, smell, touch, and ingestion (proboscis & mandibles), and housing the brain that coordinates behaviour. Worker bees have the hypopharyngeal gland for making royal jelly, drones have larger eyes, longer antennae and more brains dedicated to mating flights and spotting the virgin queen during said flight. The head of a queen is similar to that of a worker, with modifications to account for her in-hive lifestyle. For more on the insect head, read “The Head of a Bee”.
The thorax contains structures essential for locomotion and respiration, namely the six legs, the four wings, flight muscles and (part of) the tracheae. It is divided into three segments, each associated with a pair of legs: prothorax (closer to the head), mesothorax (middle), and metathorax (closer to the abdomen). A wing pair is located on the mesothorax and metathorax, and this region is enlarged in bees (relative to other insects) to accommodate their large flight muscles. For more on the insect thorax, read “The Thorax of a Bee”. (Very creative titles, I know)
The abdomen of an insect holds their organs and many defining features of a taxonomic family or genus. In honey bees there are two “stomachs” one for digestion and one for carrying nectar (the “crop”), whereas other insects get by just fine with one stomach. In females, the ovipositor is present for egg laying, whereas in wasps and bees it serves the dual function of a stinger (a really, really long stinger in the case of parasitoid wasps). Readers wanting more detailed explorations of the abdomen are welcome to search for a particular insect group’s biology. For those wanting to know about the bee abdomen, here’s The Honey Bee Stomach.
The Six Major Forms of Larval Life
The majority of insects are holometabolous, in that there is a distinct larval (“juvenile”) and adult form. The success of this strategy can be explained through the lens of competition: if both the adult and larval had the same diet/lifestyles, then you’d have the parent and offspring competing with one another for survival, which is not ideal. By having separate phases, they reduce this intraspecific competition. The juvenile’s task is to grow rapidly until adulthood, whereas the adult’s purpose is for mating and dispersal.
My entomology textbook (Gullan & Cranston, “the Insects”, 5th edition) names three larval forms on the basis of their legs: polypod (“many legs”), olgiopod (“few legs”) and apod (“no legs”). The former two denominations can be further broken down into:
Oligopod larvae:
- Campodeiform (“Campodea-shaped [a genus of primitive, wingless hexapods in the Order Diplura]).
- Examples: lacewings, ladybeetles & ground beetles.
- Appear as a flattened body with long legs, large mandibles and well-developed eyes for hunting prey.
- For example: larval lacewings hunt aphids, while adults consume nectar, pollen or honeydew (depending on the species).
- Scarabaeiform (“Scarab [beetle]-shaped”)
- Examples: various beetles: rhinoceros beetles, Christmas beetles, dung beetles or cockchafers.Have a C-shaped body, soft cuticle and slow movement.
- Larvae nest in soil, feeding on wood, dung and roots, whilst adults have various roles as pollinators or predators.
- Elateriform (“Click-beetle-shaped” [the click beetle genus Elater is Greek for “driver” or “pusher” because adults “drive” themselves into the air])
- Examples are in the click and darkling beetle species.
- Larvae live in soil, eating at roots and rotten wood (or bee hives, as with small hive beetles). Adults are surface-dwellers with an omnivorous diet, eating plant material, fruits, and the occasional soft-bodied insect.
Polypod larvae:
- Eruciform (“Caterpillar-shaped”)
- Rather-obvious examples: caterpillars of moths & butterflies.
- Have a cylindrical body, thoracic legs as well as fleshy abdominal prolegs; mouthparts specialising for chewing.
- Larvae consume leaves, adults consume floral nectar (or in some species, nothing at all).
Apod larvae:
- Apdous (“Without feet”)
- Seen in bees, ants, wasps and flies – their larvae do not move as they are placed directly on top of the food/food comes to them. No need to move around when everything comes to you.
Exceptional Larvae: The Hypermetamorphic Triungulins
Triungulin (meaning “Three-clawed”) refers to the highly-mobile first instar larvae of blister beetles (amongst other species4). In some species (oil beetles), an adult lays egg on a flower, whereby the larva hatches and awaits a suitable solitary bee. The larva emits pheromones that attracts males, fooling them into attempting to mate with it (easily manipulated horny males, many such cases). The larva instead hitches a ride on the male until he (finally) mates with a female and jumps onto her.
Back at the female’s nest, it consumes the brood and provisions (akin to cuckoo larvae), and then moults into a soft, sedentary & well-fed larva that will grow and pupate into the adult beetle form. This transition from active hunter into couch larva-tato is called “hypermetamorphosis” (or “heteromorphosis”), and I like “heteromorphosis” as a moniker because the larvae goes other lifestyle forms, hence “hetero”.
Juveniles: The Partial Metamorphosis of Hemimetabolous Insects
I conclude with a distinction between the two juvenile insect forms, larvae & nymphs. “Larvae” is the term given to holometabolous insects, those that have a distinct larval ( “kid”) and adult phases. Bees, butterflies, beetles and wasps have larval stages. Cockroaches, termites, mantids, grasshoppers and crickets do not; their young are smaller, non-reproductive versions of the adults named “nymphs”.
We’ve seen from the above that larvae and adults occupy different niches, minimising competition between the two life stages. In nymphs, this is not the case, as the diet and lifestyle and habitat resembles “mini adults”, so there is bound to be competition when times get hard. I once observed a family of cockroaches consuming the corpse of an adult roach and saw many nymphs of different sizes (as well as a few adults) partaking in the feast (please do not vomit) – perhaps having nymphal forms is not so bad when you’re a social species.
Hemimetabolism is the “incomplete” (or “partial”) metamorphosis of juveniles (“nymphs”), whereby they gradually acquire more adult characteristics following moulting. When larvae moult, they become slightly larger versions of the larval, when nymphs moults, they become slightly larger (and closer) version of the adult stage. Unlike larvae of bees or butterflies, the nymph already possesses the basic body plan: six legs, compound eyes, antennae and adult mouthparts. They differ from the adult in that there are no functional wings or reproductive organs, as well as their smaller size. There is no pupa in the final moult, and the nymph becomes an adult (“imago”) without fanfare, for insects do not celebrate adulthood.
Hemimetabolus insects are grouped in the clade “exopterygota” (“external wings”), because their wings develop externally through successive moults, as opposed to the endopterygota (“internal wings”) of holometabolous insects, since their wing & overall adult development comes from imaginal disk remodelling during in the pupal phase. Understanding metamorphosis and the jump from larval to adult remains a work in progress, as the process involves a complete reshuffling of the internal anatomy. Nymphs by contrast, are much easier to grasp, being largely guided by the actions of Juvenile hormone, whose levels decline as the nymph grows.
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Footnotes
- In general, a multivoltine species is short-lived, and even if they weren’t, their natural predators would keep them in check. Unless it is due to anthropogenic factors, brought about by humanity. Consider the unpredictable consequences of introduced species, like rabbits or various hornets (e.g. the Asian hornet Vespa velutina). Some can be positive, like the South American cactus moth (Cactoblastis cactorum) that was introduced to Australia for the (successful) control of prickly pear… but this too can be unpredictable, as when the same moth was brought into North America, it attacked the native cacti… Anthropogenic factors are unpredictable. ↩︎
- By contrasts, even the early arachnids were perpetual predators, and to date there is no “herbivorous” arachnid (although the Central American Bagheera kiplingi comes close, being “mostly vegetarian”… but no cigar). ↩︎
- From Gullan & Cranston, “the Insects”, (5th edition): similarities of insects to crustaceans are based on features of the nervous system (brain structure, how they form neuroblasts, and nerve axon developments), similarities in visual systems (e.g. their fine structure [“architecture”], the development of the lens that makeup the compound eyes [ommatidia] and optic nerves) as well as the organism’s developmental segmentation (how a single body splits into multiple units: e.g. head, thorax & abdomen in insects). ↩︎
- The parasitoid order Strepsiptera also has triungulin larvae, however they are renamed to “triungulid” in “A Dictionary of Entomology” (Gordh & Headrick). Etymologically, “triungulin” refers to “three-clawed larva” whereas “triungulid” is an adjective form meaning “triungulin-like” (or “three-clawed larva-like” – I wager it’s because Stresipteran larvae lack antennae and mandibles, despite have the three claws ↩︎
Appendix: A Comparison of Spider & Insect Anatomy/Biology
| Feature | Insects (Class Insecta) | Spiders (Order Araneae) |
| Taxonomic group | Class Insecta (Phylum Arthropoda) | Order Araneae (Class Arachnida, Phylum Arthropoda) |
| Body regions | Three: head, thorax, abdomen | Two: cephalothorax (prosoma; “fused head and thorax”) and abdomen (opisthosoma) |
| Legs | Six (three pairs) attached to thorax | Eight (four pairs) attached to cephalothorax |
| Antennae | One pair; primary sensory organs | Absent |
| Wings | Usually one or two pairs (many exceptions) | None |
| Exoskeleton | Chitin reinforced with proteins; relatively lightweight for flight | Chitin reinforced with proteins; generally thicker and more robust |
| Eyes | Usually two compound eyes plus 0-3 simple ocelli | Usually eight simple eyes (sometimes six, four, two, or none); no compound eyes (they don’t need that such complex visual apparatuses) |
| Visual resolution | Compound eyes detect movement and colour well; excellent UV vision in many species (flower detection) | Most spiders rely more on vibration than vision. Exceptions are the (very cute and tiny) jumping spiders, who have excellent image-forming vision |
| Colour vision | Often UV, blue and green; varies among orders | Usually limited, they don’t need to see much. (exception: jumping spiders possess colour vision, including red sensitivity in some species) |
| Hearing (yes, worth mentioning) | No ears in most insects; detect sound with tympanal organs, Johnston’s organ, hairs or substrate vibration | No ears; detect airborne and substrate vibrations using slit sensilla (cuticle mechanoreceptors) and trichobothria (hair-like sensory structures on the exoskeletons) |
| Mouthparts | Highly diverse: chewing, sucking, siphoning, piercing-sucking, sponging, lapping | Chelicerae with venom fangs; external digestion before ingestion for predatory purposes only (exception: B. kiplingi) |
| Food processing | Internal chewing or sucking | External digestion using digestive enzymes; liquefied tissues sucked into gut |
| Respiration | Tracheal system with spiracles | Book lungs, tracheae, or both |
| Circulation | Open circulatory system | Open circulatory system |
| Blood (haemolymph) | Usually colourless; does not transport oxygen in most insects | Contains haemocyanin (copper-containing protein rather iron [as in our haemoglobin]) in many spiders, giving oxygen-carrying capacity |
| Smell (olfaction) | Primarily via antennae containing thousands of olfactory sensilla | Primarily via sensory hairs (sensilla) on legs and pedipalps (second pair of appendages located near their mouths); no antennae |
| Taste (gustation) | Mouthparts, antennae, tarsi (e.g. house flies taste with their legs) | Mouthparts, pedipalps and leg sensilla |
| Touch | Sensory hairs distributed over body | Extremely sensitive tactile hairs covering body and legs |
| Communication | Chemical pheromones, sound, substrate vibration, visual displays, touch | Silk vibration, substrate vibration, pheromones, visual displays (especially jumping spiders), touch |
| Silk production | Rare (e.g. caterpillars, caddisflies) | Universal; spinnerets produce multiple silk types for different purposes |
| Venom | Present in some groups (bees, wasps, ants, assassin bugs) | Nearly universal among spiders for prey capture |
| Prey capture | Highly variable: herbivory, predation, parasitism, scavenging | Predominantly predatory; capture using webs or active hunting |
| Locomotion | Walking, jumping, swimming, flying | Walking, jumping, climbing, ballooning (juveniles and some adults) |
| Reproduction | Internal fertilisation; sperm transferred directly or via spermatophore | Male transfers sperm indirectly using specialised pedipalps |
| Male reproductive anatomy | Aedeagus or genital capsule | Enlarged pedipalps function as secondary copulatory organs |
| Female reproductive anatomy | Ovipositor present in many groups; genital opening on abdomen | Epigyne (external genital structure) in many species |
| Development | Egg → larva/nymph → adult; complete or incomplete metamorphosis | Egg → spiderling → successive moults → adult; no metamorphosis |
| Parental care | Variable; extensive in eusocial insects | Common in many species; females often guard egg sacs and spiderlings |
| Sociality | Solitary to eusocial (ants, bees, termites) | Predominantly solitary; a few dozen species exhibit permanent sociality |
| Defence | Camouflage, mimicry, chemical sprays, stings, armour | Venom, silk, camouflage, mimicry, threat displays |
| Sensory specialisation | Antennae integrate olfaction, touch, humidity, temperature and wind | Legs act as primary sensory organs, detecting vibration, airflow and chemicals |
| Ecological role | Herbivores, pollinators, decomposers, predators, parasitoids, parasites | Primarily predators regulating arthropod populations |