Attack of the swimming saucers

Continuing with the theme of true bugs (Hemiptera) in ponds, not all are elongate, sluggish ambush-predators - others actively chase prey and one such species is the common Saucer Bug Ilyocoris cimicoides, the common name coming from its relatively broad, convex shape.

Saucer Bug Ilyocoris cimicoides

Unlike many aquatic bugs, the middle and hind legs are only slightly modified for swimming (there is some flattening, plus clear rows of swimming hairs), and can be used very effectively for walking on land. The front legs are however more strongly adapted, not for swimming but for seeking prey.

Ilyocoris cimicoides - ventral view showing hooked front legs

The hooked legs are used for sifting through sediment and detritus, and for grasping prey which is then pierced by the rostrum - humans should take care when looking through pond-nets as the bite can be very painful and includes toxic saliva causing a sensation equivalent to a bee or wasp sting (at up to 15mm long excluding appendages, it is moderately large insect). The front legs are shown clearly in the side view below, including the broad femur (for strong internal muscle attachments) which have long grooves on the underside which presumably help to grip prey and may form slots where the hooked tibia fits into it when the leg is folded shut.

Although it may be unfamiliar if you've never been pond-dipping, it is widespread though local in distribution and can be common in some waterbodies, especially in the weedy margins of large water-bodies such as marshy dykes, old canals and lakes/large ponds.

Ilyocoris cimicoides - side view showing hooked front legs and slightly modified mid- & hind legs

Note that there is now a second species of saucer bug of the family Naucoridae in Britain - Naucoris maculatus which has been found in England and may well spread being a strong flier (Denton 2007). Happy pond-dipping!

Reference

Denton, J. (2007). Water Bugs and Water Beetles of Surrey. Surrey Wildlife Trust, Woking. [Covers a good range of species; lots of useful info and excellent photos even if you're not in Surrey].

Pond bugs - the long and short of it

As you may have noticed if you're a regular, since building my own garden pond, I have been pretty enthusiastic about all things lentic (I like that word) and this post is no exception. During a recent Southampton Natural History Society field trip (hosted by Naomi, an ecologist from Pond Conservation) to look at invertebrates associated with New Forest ponds, numerous fine creatures were found, including a couple of spindly beasts that are probably unfamiliar to most people who don't regularly wade around in the mud and reeds, pond net in hand. The first of these is the Water Stick-insect Ranatra linearis.

Water stick-insect Ranatra linearis

The first thing to say is that this isn't a stick-insect, it's a true bug (Hemiptera) in the same family (Nepidae) as water-scorpions, and as such has piercing/sucking mouthparts. The long 'tail' is its breathing tube 15-20mm long at the end of a 30-35mm long body (not including the legs) - quite a striking creature, and though locally common in southern England and wales, rarely seen as it tends to lurk in dense pond vegetation, where it is an ambush predator (like a mantis for example) grasping prey with its raptorial front legs. If caught in a pond net, it tends to look like a bit of twiggy detritus until it moves - the occasional human has been known to jump in surprise at pond-dipping events and the like...

The raptorial front legs of Ranatra linearis

Here you can clearly see how the legs can shoot out mantis-like and grab prey (often small newts and fish) which will then be pierced by the pointed mouthparts (also visible). It is able to fly - useful if a pond dries up, and they also hunt on land - and although I have never seen one doing so, apparently it looks similar to a damselfly when in flight. The eggs are a peculiar sight too - when laid on leaves of water plants such as reedmaces and bur-reeds, breathing tube emerge into the air, and the eggs look like little rows of surgical stitches; there are good photos in Denton (2007).

Moving onto something smaller, at 9-12mm long (excluding legs and antennae), the Water-measurer (Hydrometra stagnorum) is another true bug common in much of lowland England and Wales (less so in Scotland and northern England) and is especially elongate, having the longest length-to-width ratio of any British insect - even the head is elongate, protruding well in front of the eyes which can be seen about half way between the antennae and front legs.

The Water-measurer Hydrometra stagnorum

It tends to be found around the edges of ponds and streams, including bankside vegetation as well as the water surface, and are also known from ephemeral waters where they can be found under debris during dry periods. Although they are generally unwinged, individuals with wings do occasionally occur. Attracted by ripples of movement (Savage 1989), they feed on smaller invertebrates such as water-fleas (Cladocera e.g. Daphnia) and mosquito larvae, piercing them through the water surface with their mouthparts, and sometimes pulling them up through it - Fitter & Manuel (1994) includes a photo of prey being consumed. Unlike R. linearis above, the front legs are simply used for walking and are not involved in catching prey - their very regular tip-toe gait gives them their common name as they appear to be pacing out distances. There is a second slightly smaller species, H. gracilenta, but this is very rare in Britain, known only from a handful of locations in southern and eastern England. Back in the 1950s, it was apparently found in the New Forest, but the account was never published and so the alleged location remains a mystery (Huxley 2003) - to find one there would be an entomological highlight!

Although both of these species are fairly common, this is not true of all our aquatic bugs - many (most) ponds in lowland Britain are polluted to some extent, especially by nutrients from our intensive agricultural system, and this has seriously impacted the biodiversity of pond ecosystems. With both point and diffuse sources being a problem, it can be difficult to ensure good ecologicsl water quality, but advice (on managing existing ponds or creating new ones) can be sought from organisations such as Pond Conservation. Garden pond ponds are a little different as few of us have enough room for the metres of shallow muddy margins that a really good pond needs, but some of the same advice applies and they are really important oases - and excellent fun to watch as the wildlife colonises it and does its thing!

Naomi from Pond Conservation hard at work looking for aquatic invertebrates in a shallow pond in the New Forest.

References

Denton, J. (2007). Water Bugs and Water Beetles of Surrey. Surrey Wildlife Trust, Woking. [Covers a good range of species; lots of useful info and excellent photos even if you're not in Surrey].
Fitter, R. & Manuel, R. (1994). Lakes, Rivers, Streams & Ponds of Britain & North-West Europe. HarperCollins, London. Also published in 1986 as Field Guide to the Freshwater Life of Britain and North-West Europe. [Both versions are out of print, but worth tracking either down online as they can be quite cheap.]
Huxley, T. (2003). Provisional Atlas of the British Aquatic Bugs (Hemiptera, Heteroptera). BRC, Huntingdon.
Savage, A.A. (1989). Adults of the British Aquatic Hemiptera Heteroptera: A Key with Ecological Notes. FBA, Ambleside. [The standard identification guide].

Eyes in the back of my... back...

Mimicry using eyespots is widespread in nature - they are found in fish (such as the four-eyed butterflyfish Chaetodon capistratus which has them on the tail, so predators attack a non-lethal area or miss entirely), mammals (not only the 'obvious' ones such as leopards, but also the serval Leptailurus serval which has them on the backs of its ears for signalling to kittens while hunting), reptiles, birds and insects. Within the insects, butterflies and moths are probably best-known - many adult butterflies and moths have eyespots on the wings (the result of concentric pigment location around morphogenetic focus points), while the larva of the elephant hawkmoth Deilephila elpenor is famous for its conspicuous eye-like spots towards the head which are used to startle predators such as birds.

Elephant hawkmoth larva Deilephila elpenor showing eyespots

However, there are other invertebrates that show evidence of eyespots. I've previously written about bug (Hemiptera) nymphs possibly mimicing harvestmen, and today I noticed another - the common European garden spider Araneus diadematus. This is a very familiar species, often found on its orb-shaped web in gardens, and known for the pale cross-shaped marking (made from guanine which is a by-product of its protein metabolism) on the normally yellowish, orange or brown background of the top of the bulbous abdomen in females. Other common names include 'cross spider' and 'cross orbweaver', and males are smaller and less striking, though the markings are broadly similar.

Female Araneus diadematus showing the typical abdominal colour and cross-shaped marking

Male Araneus diadematus

So far, so good - but what about the eyespots I've mentioned. Well, the spiders are generally found either in the middle of their webs as shown above or tucked away in refuges at the ends of suspension silk lines. On webs they are typically head down and seen side on, either dorsally or ventrally. However, if you look stright down from the rear, a different pattern can be seen.

Female Araneus diadematus showing abdominal eyespots

To me, this is clear eyespot mimicry and makes adaptive sense - usually being head down, the rear of the abdomen is the part most likely to be presented to potential predators, namely birds, and therefore where eyespots that could startle them would be most useful. What I find more surprising is that I've never noticed this before despite having seen many specimens; more so that I can't find any other reports which suggests no-one else has either (or at least they written about it on the 'net). As ever, comments welcome!

Hands off, she's mine - mate-guarding in hoverflies

Mate-guarding is well known in some groups of invertebrates such as the Odonata (dragonflies and damselflies), and has a number of functions:

• Pre-copulatory guarding - males prevent rivals from mating, and sequester the female early in the day until the temperature has risen sufficiently to permit copulation and oviposition.
• Post-copulatory guarding - after mating, rival males are still excluded as they can displace existing sperm and replace it with their own. This may include guarding during oviposition (egg-laying) for similar reasons e.g. females are highly receptive after copulation and thus males are guarding against sperm competition, and ensuring theirs retains precedence during sperm storage.

The precise behaviour varies between species and is influenced by factors such as sex ratio, but whatever its precise form, mate-guarding has developed to prevent mating of females by rival males. For more about this behaviour in dragonflies, see for example Corbet & Brooks (2008). In other insect groups it is not always so well documented. Walker (2010) describes, for the first time, mate-guarding in the parasitic Hippoboscidae ('louse-flies' or 'keds'), while Dyte, (1988), Adler & Adler (1991) and McLean (1991) do the same for a few species of flies in the families Dolichopodidae, Tipulidae (crane-flies) and Ephydridae respectively. However, this behaviour is well documented in only a tiny proprtion of the many known fly species, so I was interested to see a pair of the honeybee-mimic hoverfly Eristalis interruptus (at least I think it's this species going by leg colour, overall form, and the pattern of the 2nd tergite in the male - the genus is certainly correct) engaging in mate-guarding behaviour in our back garden.

Mate-guarding in a pair of Eristalis interruptus hoverflies.

In this photo, the female is lower - on the ground and more-or-less still - while the male hovers (note the blurred wings) about 3cm above her, occasionally dropping down and briefly tapping her. I could find no references to this behaviour, though female hoverflies in general are known to search for suitable oviposition sites on the ground (Rotheray & Gilbert 2011) - this is possible here as it is a patch of damp. bare soil near our garden pond. However, such contact mate-guarding has been noted in the dragonfly Orthetrum coerulescens (Keeled Skimmer) and been suggested as a mechanism to induce oviposition (Miller & Miller 1989), presumably as this either renders the female receptive once more, or at least removes the male's need to guard further.

So, a rarely (if ever) reported behaviour and another example of what can be seen in even a moderate-sized urban garden if it is managed in a wildlife-friendly way. As ever, if you have seen this behaviour in hoverflies, do let me know!

References

Adler, P.H. & Adler, C.R.L. (1991). Mating behavior and the evolutionary significance of mate guarding in three species of crane flies (Diptera: Tipulidae). Journal of Insect Behavior 4(5): 619-632.

Corbet, P. & Brooks, S. (2008). Dragonflies. HarperCollins, London. [see especially pp. 226-7, 250-6]
Rotheray, G.E. & Gilbert, F. (2011). The Natural History of Hoverflies. Forrest, Tresaith.
Dyte, C.E. (1988). Mate guarding and sex ratio in Hydrophorus oceanus (Macquart) (Diptera: Dolichopodidae). The Entomologist 107: 122-126.
McLean, I. (1991). Mate-guarding in Ephydra riparia Fallen (Diptera: Ephydridae). British Journal of Entomology and Natural History 4(2): 65.
Miller, P.L. & Miller, A.K. (1989). Post-copulatiry "resting" in Orthetrum coerulescens (Fabricius) and some other Libellulidae: time for "sperm handling"? Odonatologica 10: 201-218.
Walker, M.D. (2010). Previously unreported mate guarding and 'clustering' by a nest ectoparasite (Diptera, Hippoboscidae). Dipterists Digest (2nd series) 17(2): 115-116.

Why Smurfs are like slipper limpets

Yes, I do mean Smurfs, those little blue Belgian cartoon characters... and slipper limpets are marine gastropods, Crepidula fornicata. So, why are they similar? Well, you probably know that, although there are lots of Smurfs (101 in fact), only one is female - Smurfette. Now, this could easily lead into pornographic territory (and undoubtedly has, somewhere on the Internet), but that's not what the Ecology Spot is about... instead I want to be a bit speculative and look at how this might affect Smurfs biologically if they were real...

One possibility would be that they became eusocial (like ants, bees and termites for example), with Smurfette as the only reproductive female (I assume Smurfs are viviparous, but maybe there are Smurf eggs - who knows?). However, Smurfette does not appear to be a large sedentary egg-layer (or large sedentary birther-of-live-young Smurflings), nor do there appear to be non-reproductive females rendered infertile by Smurfette pheromones. This is the case in, for example, the honey bee Apis mellifera, where the queen emits Queen Mandibular Pheromone (QMP), a pheromone set which, among other functions, inhibits ovary development in other females. So, the queen bee remains on the throne, and the princesses have to wait in line.With no other females present, and Smurfette running around actively, this seems unlikely. Instead, I think Smurfs might be an example of sequential hermaphroditism (SH).

One of the best-known examples of SH is C. fornicata. Though native to the eastern coast of North America, it has been widely introduced into the coastal waters of Europe, Japan and the NW Pacific, where it is invasive (having no predators away from its original range), competing with native filter-feeders for food. For more on its British history see here.

A stack of C. fornicata (plus a small chiton on the left) - photo by F. Lamiot, and used here under the Creative Commons Attribution-Share Alike 1.0 Generic license.

They can often be found in stacks and chains, their SH reproductive strategy meaning that the largest, oldest individuals, found at the base of  the stack are female, while the younger, smaller ones at the top are male, and some in between are 'transient'. If the female(s) die, the largest male becomes a new female.

Proestou (2005) showed that C. fornicata tended towards a 1:1 sex ratio, and that as a male's distance from a female increased, his reproductive success decreased i.e. that the males closest to the female have a competitive advantage. From this, it follows that if these males suffer a reduction in reproductive success (e.g. from competition with other males) that is greater than that due toswitching sex at a small size, then they should change. Only the lowest male in a stack can change sex, a process that takes around 60 days, during which the penis regresses and the pouches and glands of the female duct develop. If a juvenile settles on an existing stack, it develops as a male and may stay like this for up to 6 years due to pheromones released by females at the base of the stack (Fretter & Graham, 1981). Presumably the death of a female means this pheromone ceases to be produced and thus the male can change sex - another process must prevent others from changing, possibly pheromones from the new female-to-be? As there are 'transients' which complicate the picture, a pheromone gradient seems plausible.

So, although the sex ratio is different in Smurfs (100:1 rather than 1:1), an SH strategy fits well. If Smurfette dies, then as the oldest male, Papa Smurf should become Mama Smurf, with some of the others (who after all, could be 'transient' and we wouldn't know by looking at them) waiting in line.

Next post - normal service will resume!



References

Fretter, V. & Graham, A. (1981). The Prosobranch Molluscs of Britain and Denmark. Part 6. Journal of Molluscan Studies Supplement 9: 309-313.
Proestou, D.A. (2005). Sex change in Crepidula fornicata: Influence of environmental factors on reproductive success and the timing of sex change. Dissertation, University of Rhode Island.