On insect migration and fairy tales

When it comes to invertebrates, very odd ideas can circulate even in scientific circles. One of such is called 'The Pied Piper' effect, and states that many insects migrating to northern latitudes in the spring do it hopelessly, as their offspring will be incapable of making the return trip to the region of origin, where they would be able to successfully overwinter: a one way migration trip. Of course this makes no evolutionary sense. If in a population some individuals were genetically predisposed to migrate north each spring - while the rest stayed put - and the offspring of such migrants had no chance to make the return trip successfully, natural selection would select against migratory behaviour: migrants will quickly be purged out of the population, while the offspring of sedentary individuals would be more successful and propagate their sedentary tendencies resulting in a non-migratory species.
  No matter how ridiculous this idea might seem to us, it has persisted for decades, in a similar way that Aristotle believed that swallows and other birds hibernated in the mud at the bottom of ponds or that Redstarts turned into Robins in the winter. An enormous body of research has confirmed that birds are capable of migrating back and forth to distant regions and have a magnetic compass that allows them to navigate even in adverse meteorological conditions. Bird migration is now widely accepted. Could it be that our prejudices with invertebrates made us find impossible to believe that some of them also could be able of regular back-and-forth seasonal migration? It appears so. But an online early paper by Jason Chapman and co-workers, from Rothamsted Research Station demonstrates that migration in an insect is an adaptive behaviour. Migrating insects use resources that are geographically and seasonally partitioned: in the summer they are available in more northern latitudes than in the winter and a two way migration is the most profitable way to use these resources. They collected data on the Silver Y, a common noctuid moth which is known to migrate from the Mediterranean to temperate latitudes in northern Europe. This species breeds continually in up to five generations a year and cannot survive the winters of northern Europe. The very active adults (above, adults often vibrate their wings when resting and when feeding) feed on nectar-rich plants and their caterpillars grow on a range of plants, including crops, where they can become a pest.
Building on a large body of previous work of the group on insect migration, some of it I already covered in BugBlog, these researchers analysed long term moth trap monitoring data at a national scale and entomological radar data. Both sets of data agree beautifully, complementing each other. The authors estimate that between 10 and 240 million Silver Y moths migrate annually to the UK, There strong inter annual variation in numbers and three years between 2000 and 2009 with exceptionally large immigration events in 2000, 2003 and 2006, likely due to benign ecological conditions in their wintering grounds. Their results provide solid evidence of the astounding population increases - and therefore potential reproductive benefits - experienced by migrating Silver Y moths in their northern breeding grounds. As the summer progresses, adult populations increase on average four-fold and for each moth that arrives in the spring, three will start the autumn return flight. There is a clear two-way movement: northern in the spring and southern in the autumn, with the autumn migration being of larger magnitude. But are they able to migrate all the way back? Simulations of expected moth return flights using flight and wind speeds and orientation of flying moths detected using the radar support the hypothesis that most of these moths could reach to their wintering grounds in the Mediterranean successfully. The Silver Y now joins the Painted Lady and the Monarch butterfly as models of insect migration, banning the 'Pied Piper effect' to the realm of fairy tales.

More information 
Chapman JW, Bell JR, Burgin LE, Reynolds DR, Pettersson LB, Hill JK, Bonsall MB, & Thomas JA (2012). Seasonal migration to high latitudes results in major reproductive benefits in an insect. Proceedings of the National Academy of Sciences of the United States of America PMID: 22927392

The Hornet mimic Large Alder Sawfly

One of my first posts at BugBlog dealt with a strange caterpillar I was unable to ID. The caterpillar turned out to be the larvae of a rare sawfly, Cimbex connatus. The following summer I came across an adult, but it was dead and squished, so I never blogged on it. Today, just coming out of work I spotted what I thought it was a hornet on the ground. I got very excited, as I have never seen one, but my excitement turned quickly into disappointment and then into more excitement when I realised that it was in fact an adult, alive - although on its last legs - Cimbex. This is an awesome European hornet mimic, down to the abdominal pattern, size and general colouration, including the wings, and general body shape. @RichardComont at Twitter identified it as Cimbex connatus, the large alder sawfly.

Sawflies are a large and heterogeneous group in the hymenoptera, the order that includes stinging insects such as bees ants and wasps, although sawflies are harmless. The family to which Cimbex belongs has characteristic clubbed antenna. Males have enlarged hind legs and mandibles - which apparently use to fight with other males for access to females. Females have a saw-like ovipositor. They lay their eggs on specific trees or bushes and their caterpillar-like larvae feed on the leaves and come to the soil to pupate. 

I believe my specimen to be a female, as she has got an enlarged abdomen with the ovipositor sheath being visible underneath it. It measures 21 mm. I mounted it on a piece of white tack to photograph it in a more life-like position.

A side view of Cimbex connatus

A front view reveals a fierce-looking face, with large, sharp jaws and clubbed antenna.

This species is one of three large Cymbex species found in the U.K. it was regarded as extremely rare if not extinct. According to Mark Boddington:

Following a half-century hiatus to 1997, we have recent records of Cimbex connatus (Schrank) from more than ten counties in southern through eastern England, reaching as far north as the Humber. Over much of this area, C. connatus is now comfortably the most frequently encountered member of the genus, followed by C. femoratus (L.) and C. luteus (L.), the latter species in particular being infrequently seen. 

The expansion of Cimbex connatus in the UK might be linked to the increasing use of Italian alder trees, Alnus cordata, and similar species to line avenues in towns and cities.

Cimbex connatus larva found under Italian alder (23/09/2008)

The very surreal caterpillar

Something was eating the young wisteria, but we couldn't spot the culprit, until today. A large, freshly moulted Vapourer Moth caterpillar, also known as Rusty Tussock moth, Orgyia antiqua. Their odd arrangement of hairs and tufts, with neat 'shaving brushes' on their back and tail, and four feathery tufts on a grey-black and red background make it look strange indeed. And if the caterpillar is strange, the adults no less so. Adult male Vapourers are pretty day flying moths, russet coloured with a white spot on each forewing and large feathery antennae. In contrast, females are large, slow moving and flightless as they only have vestigial wings. They don't move far from their cocoon during their short lives, commonly laying their eggs on the cocoon itself. The adults do not feed, but the caterpillars are generalist feeders and use a wide range of bushes, and compensate for their mothers immobility by dispersing widely. I have seen the caterpillars from June to August. After her white bowl treatment, this one was moved to the large cotoneaster, where I have seen them before.

More information
UK Moths page.

Migrant Common Darters

At this time of the year, my garden - which hasn't got a pond - hosts occasional dragonflies as they migrate to warmer climates. The most regular one is the Migrant Hawker, which often hunts in small parties 2-3 m high, occasionally stopping to rest hanging from branches. Rarer species are the Darters. In the last few days, I have watched female Common Darters, Sympetrum striolatum, resting and hunting. The first day it banked atop my parasol and I couldn't get good shots. Today I was luckier and the darter sat on the fence. With the help of my new Canon Powershot G12 flip out LCD display, and perched on a chair, I could hold the camera high enough to get its portrait.

This species, as other darters, is known to migrate regularly. David and Elizabeth Lack reported their pass south in the Pyrenees together with migratory birds, butterflies and hoverflies in October 1950 and reviewed the information available on dragonfly migration until then. Today, although the morning was warm with sunny spells, a large front with thunderstorms passed through all the afternoon, so it could be that the dragonflies were moving away from the coming rain. 

Common Darters are known to form occasional irruptive, massive migratory swarms, although the reason for these are not well known. A large irruptive swarm of this species was reported in Ireland in August and September 1947 by Cynthia Longfield from the British Museum. She compiled the letters of many observers amazed by the sudden, unprecedented invasion, including interviews with schoolchildren, which helped document the spatial extent of the invasion. As, she was visiting Ireland at the time, she identified the dragonflies and further documented the invasion with her observations. She concluded that this mass invasion event probably came from Spain and Portugal, as no invasion was reported in France or England at the time. She calculated that the dragonflies must have flown over the sea overnight. It is hard to imagine what being there at such massive influx event would have been:

'They were flying low in compact formation. . . . The flight was orderly, in a northern direction, as if they were intent on reaching some known destination.'
'They seemed more to be drifting with the breeze than flying, but were keeping to a straight course, without zigzagging or undulating, coming in from-the sea to the south and going north. What was so remarkable was the witnessing of the actual arrival and passing of the swarm.'

'They seemed to be coming in from the sea and flying about two feet above the ground, though some were higher, to about eight feet up. All were flying in the same direction, northwards. It seemed a never-ending stream, but we had to move on, and passed out of their track, so I could not tell the duration of the flight. They flew considerably faster than we were walking. The density was about one foot, or rather less, apart.'

For the next day, 3rd September, Mr. Allen writes: 'Tuesday's invasion was nothing to Wednesday. Wind south-west. Before noon they commenced and continued until evening. They not only came singly, drifting as on Tuesday, but in great, thick black masses, like dark clouds. They really frightened the people.'

Here he found 'the people looking out to sea, and I saw a sight which gave me a fright at first. I heard a hum like bees and yet not quite like bees. The next thing the whole sky filled with large flies that resembled aeroplanes, millions of them'

 Cynthia hypothesizes that the reason these dragonflies were emigrating was the drying up of the marshes where they regularly bred. That these dragonflies somehow were prevented from completing their reproductive cycle in the region of origin is illustrated by the fact that many of them paired on arrival, and some were even observed laying eggs on the sea. Although these spectacular irruptions appear to be extremely rare occurrences, they illustrate that the more regular, seasonal migratory behaviour of some dragonflies, can possibly be exacerbated by unusual weather conditions in their region of origin - in a similar way that occurs with Painted Lady irruption years.

More information

Cynthia Longfield (1948). A vast immigration of dragonflies into the South coast of Co. Cork The Irish Naturalists' Journal, 9 (6).
David and Elizabeth Lack (1951). Migration of Insects and Birds Through a Pyrenean Pass. Journal of Animal Ecology, 20, 63-67 DOI: 10.2307/1644

The Batman hoverfly

Surely, this is not an accepted English name for this common and widespread hoverfly, but the reason that this species is easy to identify is its distinctive 'Batman' logo on its thorax. This large female fed on the fennel with two other individuals. I find it amongst the most handsome of the British species. The larvae is of the rat-tailed maggot type, like its relatives, Eristalis, the droneflies and Helophilus (the 'Footballer hoverfly') and it typically breeds in rot holes in trees where leaves decompose.

Drama under the barbeque

This morning, while watching the hoverflies feeding on the fennel I realised I wasn't the only one interested in them. A female digger wasp Ectemnius, was poised on a cherry leaf, attentively following the action, and at one point the spotted a suitable hoverfly victim and attempted a capture.
 In the afternoon I came across Ectemnius again. Another female inspecting potential nest holes in a bee hotel, going in and out of them.

 I decided to sit and watch the action, faintly hoping the wasp would start digging a nest. I noticed the characteristic carded silk threads of Amaurobius spiders (top shot), and thought that spiders and digger wasps must come across each other with some frequency as female wasps inspect nesting sites. Then the wasp moved to the log pile under the BBQ and walked in and out amongst the logs. She appeared to have walked onto a silk thread and a false widow spider, Steatoda bipunctata, promptly came out of her refuge. A fight ensued, the wasp trying to bite or sting the spider, the spider spreading silk on the wasp and retreating, and repeating the procedure as the wasp became more and more entangled and buzzed intermitently in her futile attempts to get herself free.

 
The commotion got an Amaurobius out of her retreat, and at some point both spiders were attacking the wasp, although the Steatoda managed to secure it and, when the wasp stopped fighting, the spider cut free the threads that attached the prey to the log, and pull her to the safety of the retreat.

Early stages of the fight

The Amaurobius' legs are visible behind the abdomen of the wasp in the shot above, while the Steatoda secures the jaws of the wasp with more threads of silk

The wasp front end is now tightly wrapped on silk, now the spider focuses on the rear end

The spider drags the wasp to her retreat

 I watched all this with amazement and struggled to get decent photos as the action happened almost behind the log. The whole sequence of events happened within 15 minutes!

Candy-stripe spider

 I found four Candy-stripe spider, Enoplognatha ovata, females guarding their lovely blue egg sacs under the lid of my garden waste bin. The spiders have probably been transferred to the bin with branches cut from some bushes, and had moved with their egg sacs to the drier lid. This one floated down from a thread of silk, and I brought it home to take her portrait on the white bowl. Another thread of silk attached the female to its egg sac, as can be seen on the following side shot:

Although these are small spiders, about half a cm long excluding the legs, they are powerful hunters, and they are able to subdue wasps, honeybees and even the aggressive Wool Carder bees, which they capture on their loose, disorganised web under flowers or leaves, where the females guard their egg sac:

Enoplognatha with its male Wool Carder Bee prey

this one with wasp

with Episyrphus balteatus hoverfly

This one hiding underneath a Feverfew flower got a honeybee

This spider shows a striking color polymorphism, which is genetically determined in a complex way. I have found the three colour forms in my garden. The abdomen can be plain pale yellow (variety lineata, top shot), yellow with two dorsolateral pink stripes (var. redimita):

or yellow with the dorsal surface of the abdomen entirely pink (var. ovata), the rarest in my garden:

In addition to the colour polymorphism the presence and number of black spots is also variable, as it can bee seen in the above photos. This species also occur on both coasts of the USA, presumably introduced from Europe. In the UK, there is a very similar species, also colour polymorphic, E. latimana, which has been recorded mostly in the south of England and it appears to be less common than E. ovata.

Flies dancing on water

Approach a pond in July or August and crouch down. Focus on the surface of the water, checking the floating leaves or debris by the water edge and you will likely see jittery, shiny green flies. Get comfortable and watch them for a while, it takes some time until you can transport yourself to this small, alien world. These are long-legged flies, Poecilobothrus nobilitatus. They like to rest on floating objects or the mud on the edge (above), but they are also able to walk on the water surface. You will notice that some flies, the most active ones, have smoky wings with bright white wing tips - they are the males (top shot). Clusters of males may dance around single females, which lack the white wing marks (below).

Female walking on the water surface

To start courtship, the male focus on a female and approaches her, then he opens his wings in straight angles to his body and buzzes them quickly before closing them again, the wing-waving display, which is just a component of their complex and spectacular courtship. 

 Here several males circle a female wing waving, although the wings are barely visible as they are buzzing them. The three males court a female, which is watching the male in the right.

A male in the middle of wing waving

I took a little clip of their courtship earlier today.

The details of the courtship are a combination of wing waving and aerial acrobatics too fast for the human eye to follow, but they were described in detail by M.F. Land, which examined an hour of videos of courting flies in a pond in Sussex. The following figure from his paper represents the longest bout of filmed courtship, which was interrupted by the male chasing another male. Wing waving upon approach to a female is followed by hovering in front of her and flying over the female and back, and more wing waving. Courtship is often interrupted by chases to other males and then returns to the same female to carry on.

(from Land, 1993)

UPDATE: Thank you to Morgan Jackson for pointing out that this fly was one of the species in the Name a Species 2012 competition and won the lovely English name of Semaphore fly.

More information

Land, M.F. (1993). The visual control of courtship behaviour in the fly Poecilobothrus nobilitatus J Comp Physiol A, 173, 595-503 DOI: 10.1007/BF00197767

Spotty Eyed Hoverfly

Occasionally, I wonder if this blog can be long lasting, given the subject is my garden invertebrates. Can I regularly find something worth writing about without repeating myself? These doubts, however, are always short-lived, as I am time and time again surprised by a new species or by regular ones behaving in ways I haven't observed before. This afternoon, before the skies opened and rain started falling after a couple of dry weeks, I watched hoverflies, always plentiful this time of year. The flowering fennel and yarrow, both with flat umbels and many small flowers with easy to reach nectar were buzzing with them. A small, stout hoverfly fed on the fennel. I didn't recognise it and I was amazed by its unusual spotted eyes. Taking photos was tricky, as the slender fennel stems swayed on the wind. I took many, and discarded most, but these are the best I managed.
 The wonderful Garden Safari website helped to identify the species as Eristalinus sepulchralis. Unfortunately, despite having browsed Stubbs and Falk, the plate in which it is pictured is at the same scale than other much larger droneflies, and the characteristic eyes were not apparent in the small drawing. Also, as the drawings are based on pinned specimens, and eye colour fades on dead specimens the contrast between the yellow background and the darker spots is lost. This species is characterised not only by its unusual eyes, but also for the striped thorax, metallic bronze sides of the abdomen. Despite it being my first sighting, this is a common and widespread species, with maggots of the rat-tailed type that develop in wet areas. Adults overwinter and feed on a range of flowers.