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Insects and arachnids morphological comparison

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Insects and arachnids morphological comparison

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Body segments: insects have 3 main body segments (head, thorax, abdomen) while arachnids have 1 or 2 (cephalothorax, abdomen) – with more segments comes a more adaptable body plan as there are more possible combinations of specialized functions of each segment, hence insects can diverge and speciate at a greater rate than arachnids.

Mouth parts: arachnids are limited in their feeding by their chelicerae mouthparts, which are relatively limited in their adaptability, which require that they liquify their prey before consuming it, so they are confined to hunting prey. Insects have a much greater mouthpart diversity which allows them to consume a much wider variety of food types between species as more insects can specialize to consume a unique food source.

Flight: many insects possess wings which allow them to fly and colonise a wide range of unique habitats, whereas arachnids do not possess wings, limiting them to crawling and climbing. While arachnids can still occupy a wide variety of habitats, this is dwarfed in comparison to the habitats and niches occupied by insects. Access to novel habitats gives rise to a greater rate of speciation, hence why insects are far more speciose than arachnids.

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Important to recognise human aspects of this question

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Important to recognise human aspects of this question

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Insects have three body segments while spiders only have two, This allows insects to diversify their body plan in a greater variety than spiders. Insects also have wings and a variety of mouthparts (mandibles, sucking mouthparts, proboscis) whereas spiders exclusively eat liquid from a sucking mouthpart and must first capture their prey and have the time to consume it. Insects can also eat a variety more food types from seeds, nectars, and other herbivorous items as well as being omnivorous, insectivorous, predatory, and parasitic. I’d like to add (as I think it an important aspect to recognize and not to forget) that humans designed the “boxes” we place these organisms into. Just because these two groups are on the same taxonomic level doesn’t mean we have to categorise them that way. Therefore I personally feel that explaining how specious a taxonomic group is, is only a valid question and worth answering if we do so through the lens of understanding that our definitions of them and our need to categorise, is a large part of the answer.

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Mouthpart and wing morphology has led to tremendous diversification in insects, allowing them to fill many niches that are currently unavailable to spiders. The sucking mouthparts of arachnids severely limit individuals to a carnivorous diet. Insect mouthparts however, have undergone significant adaptive radiation, involving sucking/lapping/crushing/piercing mouthparts to utilise more niches. Similarly, the evolution of wings in insects allows access to new spaces/habitats, furthering the development of new species.

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The morphology of insects has allowed a higher level of adaptive radiation when compared to arachnids. Morphological differences in insects such as diverse mouthparts, presence of wings, metamorphosis, and short life cycles create an ability to diversify by filling new niches like new habitats and food resources. Arachnids are limited by their simple mouthparts equipped for predation and liquid foods as well as their restrictive ability to disperse to new habitats. Insect mouth parts are more complex and have evolved in different species to consume many food types, from leaves (herbivorous) to blood (predatory). As arachnids have one life cycle it also means they have to compete against their own young for food resources where as insect larvae consume different foods to adults. Arachnid can have much longer life spans than insects, meaning their ability to evolve is slower

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Morpholoical differences that make insects so speciose compared to arachnids

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Morpholoical differences that make insects so speciose compared to arachnids

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- Many insects have the ability to fly but arachnids do not
- Insects have mouthparts that allow them to ingest a greater diversity of foods whereas spiders can only suck the fluids of prey
- Some insects have an exoskelton which allows them to live in dryer environments
- Some insects have the ability to breathe underwater which would allow them to obtain new food sources and survive in wetlands

These points allow us to see taht insects can survive in a wider range of environments and obtain a wider range of food sources compared to arachnids.

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The variety of insects compared with arachnids, is vastly different and more specious. The main reason why insects are more successful than arachnids is due to their many morphological differences that have made them radiate into all habitats. Insect mouthparts is a study in itself, containing everything from biting, sucking, piercing and lapping. Compared to arachnids, spiders only have mouthparts for sucking fluids from flies, and smaller bugs. Insect mouthpart diversification allows them to feed on a variety of prey such as mosquitoes to humans, caterpillars to leaves, butterflies to nectar. Another adaptive radiative element is an insects life cycle. Containing multiple stages from larvae to pupal to adult. An example is mosquitoes. Their larval stage is dominant in aquatic environments, with their final adult stage in the air. This reduced the competition for food between adult and larvae, where as compared to spiders, their body plan is consistent throughout their life cycle. Overall, insects show more morphological differences than spiders which allows them to open up new niches within the environment.

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Diversification of multiple mouthparts of insects have allowed almost all food sources to become available to insects. Wings have allowed colonisation of aquatic and aerial habitats while a shorter lifespan in insect species has made evolution occur at a faster rate able to keep up with current environmental change.

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Differences in morphology allow insects to occupy more diverse niches than arachnids

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Differences in morphology allow insects to occupy more diverse niches than arachnids

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Differences in morphology allow insects to occupy more diverse niches than arachnids, particularly due to presence of extra sense organs, wings, the ability to metamorphose and a wider range of mouthparts.

Insects differ morphologically from arachnids in a number of ways. They have a different number of legs (six as compared to eight), their body plans are separated into three tagma rather than two, insects have antennae and wings and also undergo metamorphosis. Arachnids do not have antennae or wings and grow into their adult shapes rather than metamorphose.

As a result, insects are hugely diverse and can occupy a higher range of environments than spiders. Their ability to fly allows them to occupy aerial environments, while ability to metamorphose means that nymphs can occupy aquatic environments and then undergo changes to inhabit a terrestrial environment (e.g. Anopheles latens or other mosquitoes). Aside from using web for parachuting, arachnids are mostly limited to terrestrial land-dwelling or in trees.

Development of mouthparts in insects also allows them to subsist on a more diverse range of foodstuffs. Arachnids tend to have sucking mouthparts, and there is less diversity of type of mouthparts.

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Limitations of spiders, with respect to insects

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Limitations of spiders, with respect to insects

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- Absence of wings, less chance for movement
- Simplified mouth parts compared to insects, less room for specialisation
- Exclusively predators, more competition between species
- No metamorphosis, both young and mature spiders would come into competition with each other

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Insects commonly have three body segments, whereas spiders have two tagma 

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Insects commonly have three body segments, whereas spiders have two tagma 

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Adaptive radiation means insects have evolved to fill many niches, such as butterflies as pollinators, and mosquitoes as predators – each with varying morphologies to better suit their lifestyles, an insect’s ability to adapt to varying conditions allows them to adopt diverse forms. Insects also have shorter life-spans than spiders, with some insects only living for a few days, while some spider species can live up to 20 years, this means evolution has taken place on a more accelerated timescale for insects.

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- Insects have 6 legs, while arachnids have 8
- Insects possess wings that allow for larger area locomotion and habitat colonization (therefore leading to adaptation)
- Insects have a wider variety of mouth parts (maxilla, labrum, labium, mandible) compared to the spider chelicerae.
- Insects feeding methods and prey variability would also lead to changes in species in a larger number then that of arachnids

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Although insects and arachnids share many morphological traits, they do have some differences that may be the cause of some big differences in their rate of speciation.
Insects have wings which allow them to expand their habitats quite easily resulting in allopatric speciation.  Insects have 3 tagma which can result in diversification of their mouthparts to adjust to new habitats. This can occur over a shorter timescale as insects have shorter lifespans than arachnids and will result in more generations over the same time span.
Arachnids are limited with their movements to how far their legs can move them and the areas they can move into.  They are successful predators and diversification may not have resulted in an increased success in reproduction reducing the likeliness of it being a successful adaptation. Arachnids only have 2 tagma reducing the availability of areas to diversify.
These differences between the two taxa may result in why adaptive radiation may be more successful in insects when compared to arachnids

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Exoskeletons in insects would have protected them more against desiccation. This would have assisted them in radiating to fill different ecological niches

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Exoskeletons in insects would have protected them more against desiccation. This would have assisted them in radiating to fill different ecological niches

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Due to insect's presence of wings, they can move to new locations at a much higher rate which causes an increase in the development of differing species. Their variety of mouthparts also allows for a variety of feeding habits so they can utilize many different food sources so more species with specific feeding methods can emerge.

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• Insects have six legs, arachnids have eight
• Insects have compound eyes; arachnids have simple eyes
• Insects have antennae; arachnids do not
• Many insects have wings; no arachnid have wings
• Insects have mandibles; arachnids have chelicerae
• Insects undergo some form of metamorphosis; arachnids do not

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Insects have a diverse array of mouthparts which allows them to feed on different prey, where as spiders only have sucking mouthparts and are limited in what they hunt.

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Insects are the only invertebrates with wings. This enables them to fly and colonise new habitats.

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- Insects have up to four wings, which gives them access to a whole new territory (more or less) inaccessible to spiders. Filling these niches creates diversity.

- Insect mouthparts more “refined” (labrum, labium, mandible, maxilla) than chelicerae, allowing for more ways to develop specialisations.

- Insect metamorphosis allows for restructuring or reorganisation of body parts to better adapt to the specific need of each life cycle (e.g. growing, reproduction). Spiders undergo a series of molts, however, their body plan stays the same. This multi-form existence gives insects even more ways to diversify.