Spiders and insects: Evolution’s Tom and Jerry chase?

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Among the many interesting points raised during our recent Diversity and Evolution of Insects module was the idea that spiders and insects may have been involved in a sort of cat and mouse pursuit through the fossil record.

It’s certainly the conclusion David Penney reached in his 2004 paper looking at insect and spider family diversity over geological time. It’s suggested here that the rate of exponential increase in diversity was the same for both groups, and that one’s line of rapid diversification – known as radiation – followed the other.

Both insects and spiders tend to be linked with the history of flowering plants, but interestingly, the study also suggests that the major radiations of both these classic antagonists were out of the way a good 100million years before the flora joined the party. This being the case, the arms race began without the need for the habitats that we’d probably associate with the modern tussle of flying insects and web-weavers.

Co-evolution has been proven to be more likely when there’s a direct interaction between groups, and at least one dependency, so the idea of a hunger so profound it transcended the ages certainly sounds plausible. Yet it’s not a view universally shared.

It’s hard to conceive of the struggle between insects and spiders without thinking of webs – and the diversity of arachnid species is linked with the diversity of web design. But in the poetically-titled Tangled in a sparse spider web, researchers at the University of Barcelona muddy the waters of the ‘insects lead, spiders follow’ story of speciation.

They make a case that the diversification of spiders and their web-building approaches was all about moving to different habitats and making use of food resources in an increasingly structurally complex world. To be clear, it’s abundance of prey, they say, that was more significant in driving a species-defining approach to webs rather than its diversity. Loads of flying insects, yes, but not necessarily loads of different ones. They also make the case that the explosion of orb-webs couldn’t have happened at the same time as the insects were on their fiercest period of diversification.

Searching for trends through what remains of the species that have been here and gone is a notoriously tricky business – something that is more than acknowledged by the authors of the different theories offered here. Missing data is one of the foremost problems with scouring the past for clues that may illustrate a trend, while the ‘family trees’ considered in invertebrate evolutionary studies are often complicated and controversial; subject to different interpretations and revisions.

So, has predator chased prey through the ages, or are things a little more complex than that? Well, this is science – never the easiest place to get a neat narrative from. So while you can find shadows of Tom and Jerry, Road Runner and Wile E. Coyote, Bugs and Elmer and the rest if you trace the lineages of Arachnida and Insecta, pinning evolutionary trends on a hunter-hunted analogy alone probably won’t quite cut it.

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Parental Care in Insects

The concept of parents looking after their babies is easily recognisable. We were all cared for when we were younger (even if not by our biological parents); fed, clothed and housed. We recognise the same urge when we see a cat cleaning her kittens or a bird collecting nest materials, but do we see it in insects? Well, more than you might think.

Only about 1% of insect species show parental care; selection pressure favours lots of offspring, effectively limiting parental care to species which produce fewer young. Parental care can mediate the transition from solitary to group or family living, both for an individual and on an evolutionary scale. It is likely to have developed as an altruistic trait to enable parents to better ‘pass on their genes’ by improving the survival of their offspring, even at the cost of their own energy, food, or even future reproductive opportunities. It may be evolved in environments with high selective pressures such as predation risk or reduced food availability.

Some insects show pretty much every level of sociality that you can think of, so here’s a walk-through of what insect parental care looks like. Through egg care, larval care by one of both parents., and the formation of family groups, this is the insect guide to good parenting:

Egg care

In some species, parental care starts before the young even leave the egg. Female earwigs groom their eggs to remove harmful mould spores and secrete symbiotic bacteria onto the larvae which are both antibiotic and anti-fungal. One study found that only 4% of European Earwig (Forficula auricularia) eggs hatched when they were left untended, as opposed to 77% for tended eggs. Mothers do the bulk of the offspring care but for some species the father takes the burden.

Male water bugs (Belostomatidae) brood the fertilised eggs on its back until they hatch. Carrying the eggs around makes the males more vulnerable to predation and hinders their foraging, making a large energetic expense to rear their offspring. It also stops the male from being able to mate again until the eggs have hatched.

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Male Giant Water Bug carrying his brood (Matt Tillett)

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Female European earwig, (Foricula auricularia) with egg brood and nymphs (University of Florida)

 

Uniparental care

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The Australian Hornet (Abispa ephippium) with her nest (Ian Sutton)

Potter wasp (Eumeninae) females build small clay nests for their larvae, bringing them food, defending them against predators, repairing damage and cleaning debris from the nest. Males play no role in the larval care but do patrol nest sites to find females to mate with.

 

 

Biparental larval care

Burying beetles (Nicrophorus spp.) take it up a level with both parents caring for their brood. Not only do they stock their larvae’s underground nest (or ‘crypt’) with a decomposing carcass (yum), but they also feed them regurgitated meat if begged. If there isn’t enough carrion to go around, parents will cull the most demanding larvae; whilst this might seem harsh, it ensures the survival of their less needy siblings. Although the parents don’t form the monogamous pairs often seen among vertebrates, they will stay together until their larvae reach adulthood.

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Nircrophorus vespilloides individual feeding its young (Dr Clare Andrews)

Family groups

Cockroaches might not initially seem the most likely parents given their occasionally cannibalistic tendencies but show some of the most comprehensive parental care of the insects. The females of some species are viviparous, gestating their offspring under their wings and producing a protein and carbohydrate rich ‘milk’ to feed their young nymphs until they are old enough to be ‘born’.

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A pacific beetle cockroach (Diploptera punctata) ‘giving birth’ to nymphs. This species also produces cockroach ‘milk’ (Emily Jennings)

Both cockroaches and burying beetles, unusually for insects, form family groups. In cockroaches this is because nymphs need to receive the gut protozoa essential for digesting cellulose from their woody diets. They lose the protozoa after every moult meaning that to ensure the nymph’s survival the adults have to stay with their offspring until they reach adulthood.

It is thought that this feeding was the key which allowed ancestors of modern termites to become eusocial. Termites are very closely related to cockroaches and these family groups expanded and evolved to become eusocial colony organisms. Living close together with millions of their ‘siblings’ allows termites to be sure of security and a food supply and allows traits like monogamy, foraging and nest inheritance to be developed. The switch from parental to sibling care is thought to have led to social behaviour forming in ants, wasps, and bees.

Evolution of sociality

But what is the glue holding these parents to their offspring? It’s surely not the big eyes, fluffiness, and helplessness that draws us to babies, kittens, and ducklings- even entomology students would struggle to call a cockroach nymph cute. It seems insect ‘families’ are reliant on pheromones as a recognition mechanism. Earwig nymphs’ pheromones reflect the quality of the food they’re being given to influence their mother to provide more food if needed. Cockroach nymphs use similar pheromones to aggregate with their parents and siblings and are able to distinguish non-siblings.

Brood care is thought to also have driven formation of families and social groups in vertebrates. The evolution of parental care in insects can be a model for the evolution of parental care in birds, fish, and of course mammals. It seems that you might have a lot to thank cockroach milk for your survival to adulthood.

 

An Update (Part 2)

Welcome back peeps! Here’s the second part of the overview/highlights of what we’ve been getting up to so far on the course:

Module 2: Diversity & Evolution of Insects

This module was a nice transition from the previous module content-wise. The first day was a mixed bag, it started with a lecture from Prof. Simon Leather (@EntoProf) on the history of entomology as a subject and insect paleontology (come on, who doesn’t love a bit of Meganeura spp.). Followed by Dr. Andy Cherrill giving a lecture on intraspecific variation. Theeeeen, back to Simon, with a lecture on the super weird, awe-inducing and ever so slightly ridiculous aphid life cycle. The day concluded with the first guest speaker for this module: Professor Tony Dixon! He gave us a lecture on aphid thermobiology and coccinellids (ladybirds, namely on generation time and their usage in biocontrol). He was Simon’s PhD supervisor! It was a privilege to be lectured by someone who has been in the game for so long, is still publishing research and has taught one of our lecturers. The next day was also a healthy mix of topics, covering soil biodiversity to aquatic insects and estimating insect species diversity.

Leading on from the previous day, we had a lecture on Acari (ticks and mites). The study of non-insect arthropods meshes nicely with entomology. As entomologists, it is important for us to be able to identify relatives and to understand their ecological interactions. The rest of the day was full of the mighty Odonata! Starting with a series of lectures from guest speaker Steve Brooks (once again, from NHM) on the identification of British Anisoptera (dragonflies) and Zygoptera (damselfies). The afternoon was spent gleefully identifying odonatans using their larval exuviae with The British Dragonfly Society’s Shropshire County Recorder, Sue Rees Evans!

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The larval exuviae of the Southern Hawker (Aeshna cyanea). Dragonfly larvae are predatory and possess a labial “mask”, a modified labium tipped with pincers. The mask is fired out to grab and immobilise prey. 

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The larval exuviae of a damselfly. The appendages on the rear are called lamellae and they aid gas exchange.

With Odonata checked off the list, we had a day dedicated to an assortment of insect orders with Dr. Mike Copeland. To name a few, we covered the Phasmida, Dermaptera and Neuroptera. The following day started off with a practical session in which we unleashed the fury of lacewing larvae onto some chubby mealybugs; a little taster of what is to come in the Commercial & Practical Biological Control module!

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A lacewing larvae chowing down on a mealy bug. They are voracious predators with specialised mandibles used to extract the bodily fluids of their prey.

The module and week ended on a mellow note, with another chill session of pinning and curation. Practice makes perfect!

Module 3: Experimental Design & Analysis

Being able to design an experiment to test a hypothesis and then analysing the acquired data using the appropriate statistical analyses, holds fundamental importance in science. Once again, the course is full of people with varying levels of experience in different areas, and statistics is no exception. The module reinforced the importance of a robust experimental design, and introduced the cohort to the statistical software R and how to run a range of tests using it. Of course, I would rather have fun practicals over this in a heartbeat, but you can’t replace bread and butter with more filling and expect to have a sandwich! Having just finished this module, we start the Commercial & Practical Biological Control module on Monday! *crowd cheers* HUZZA!!

Soooooo…that’s it from me for now! Linzi will be posting an article on Tuesday on insects which survive in extreme environments and their adaptations to the hostile conditions they live in.

Until next time!

 

By Aqib Ali  (Twitter:@EntoAqib , Email: Aqib1996@hotmail.co.uk , Linkedin: Aqib Ali)

MSc Entomology Twitter: @EntoMasters

Insect flight – an evolutionary development that shaped the world

Flying animals have had a major impact on nonflying organisms. Briefly consider the ecological and evolutionary interrelationships between pollinators and flowers, or between mosquitoes, the parasites they transmit and humans. Even a cursory glance at the manifold relationships flying insects have with all other forms of terrestrial life evaporates any doubt whether the world would be a very different place if they had never evolved.

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