Archive | May, 2010

Stick Mimic

31 May
A stick grasshopper on a flower.

A stick grasshopper on a flower.

Look twice.  That’s not a stick.  It’s not even a walking stick.  It’s a grasshopper!

The stick grasshoppers comprise the family Proscopiidae, and are unique to South America.  Aside from their jumping hind legs, their heads are very distinct from that of a walking stick: elongate, with a mouth near the bendable neck, and eyes further along the length of the head.

These guys were all over Argentina and fun to watch.  This particular fellow was posing on a flower all by himself, so I had to snap a picture.  Here’s another stick hopper, this one particularly cryptic:

A well-camouflaged stick grasshopper in Argentina (Proscopiidae).

A well-camouflaged stick grasshopper in Argentina (Proscopiidae).

To Bee or Not to Bee

28 May
A bee-mimicking hover fly visits a flower.

A bee-mimicking hover fly visits a flower in Argentina.

If you thought this was a picture of a bee, look closer.  A few key characters will give this mimic away.  Bulging, compound eyes take up most of the head.  The antennae are two short stunted nubs at the front of the head between the eyes.  The ‘waist’ or connection between thorax and abdomen is broad not narrow.  And most diagnostically, flies have only one pair of wings, not two — the hind wings being modified into two short knob-like structures used as counterweights in flight.  Thus the fly order name Diptera, “two wings” (two wings make up one pair).

This particular fly is a member of the genus Syrphidae, the hover flies or flower flies.  Many members of this group are bee or wasp mimics, as well as important pollinators of flowers.  As their name suggests, these flies are especially well adapted at hovering (aiding in flower visitation)  a skill which the males of some species use to impress females.  Such males stake out a spot in the air and attempt to remain ‘motionless’– an impressive feat for a tiny insect easily buffeted by wind currents.  The male who best holds his postion for the longest period of time is considered the sexiest.  The BBC series Life in the Undergrowth has an excellent segment on this behavior.

Exponential Orbweaver

24 May
A gregarious orbweaver spider

A gregarious orbweaver in Argentina.

Another arachnid post.  These spiders were one of the most striking arthropods we encountered in Argentina.  The females of these colorful spiders are nearly palm sized.  Communities of these gregarious or sub-social spiders form their webs in large groups, thus filling a small area with many, many huge spiders.  This lead to some interesting encounters.  Myrmecologists scouring the ground for fire ant nests aren’t always particularly aware of, say, what might be going on with spiderwebs in the trees they’re walking through.

An aggregation of gregarious orbweavers in Argentina

It is very disconcerting for the spider-wary to accidentally stumble into one of these clusters of giant spiders.  Luckily, I did not find them to be particularly aggressive.  The webs are sturdy, however, and capable of straining fairly large flying prey from the air.  We even saw a bird become trapped in one of these webs for several minutes before it beat its way free.

A bird caught in the web of gregarious orbweavers (photo by Dr. Shawn Wilder).

In truth, I haven’t been able to find out much about these spiders.  Does anybody know of any good references?

Spider Hider

21 May
Spider in refuge.

A spider in its refuge.

Still more from Argentina.  Spiders are not insects so this particular arthropod has 8legs2many.  But he gets on the site anyway.  That’s also my excuse for not being able to provide a better ID for this guy.  If I had to take a stab at it, I’d say jumping spider.  But if you know your spiders better than I do I’d love to hear it.

Many spiders use their silk not for spinning webs and capturing prey, but rather for creating a comfortable hiding place, called a refuge.  Depending on the type of spider and environment, they may weave an entire cocoon-like nest or incorporate materials from their surrounding environment.  Alternatively, they may line burrows or pre-existing cavities with webbing to make the walls of their home easier to climb.  In this case, the spider has used silk to fold over a leaf to create a hiding area.

Experience has shown me that picking apart bundles of silk found in the wild will sometimes reveal a cache of lovingly swaddled spider eggs or perhaps a cocooned insect pupae, and sometimes an angry/terrified spider.  Fun!

Mercury Vapor Lamp and Light Sheet

17 May


Light sheet in the woods at night.

Light sheet in the woods at night.

New page up in the techniques section.  Please check out my discussion of the use of light-sheets to attract nocturnal flying insects.

Find more techniques for insect collecting and rearing in the techniques section.

Damsel and Prey – the Narrow-Winged Damselfly

14 May
A damselfly with captured leafhopper.

A damselfly with captured leafhopper.

Found this damselfly in Argentina, munching on a tiny leafhopper.

Damselflies (Odonata, suborder Zygoptera:  “paired wings”) can be distinguished from dragonflies by the shape of their hind wings, which are similar to their forewings, narrowing at the base like a petal.  Dragonflies (Odonata, suborder Anisoptera: “unequal wings”) have a hind wing with a broad, lobed base.  Additionally, as their names would suggest, dragonflies tend to be larger, and thick-bodied, while damselflies and small and slender.

Both dragonflies and damselflies are agile predators, snatching flying prey out of the air.  Damselflies have a special adaptation for this.  They use their forward angled legs to form a ‘basket’.  Long bristles on the legs complete this structure, allowing the damselfly to sieve prey from the air.  Once caught in the ‘basket,’ the prey is then transferred to the jaws of the damselfly.  In this case, the damselfly has caught a leafhopper, family Cicadellidae.  A close look will reveal that this family tends to resemble tiny hopping cicadas, a relative in the hemipteran suborder Auchenorrhyncha.

Leafcutters on Parade

10 May
A trail of leafcutter ants carrying plant clippings.

A trail of leafcutter ants carrying plant clippings.

These are leafcutters of the genus Acromyrmex, like the individual foraging on a tree in the previous post. Workers of these ants clear obstacles and debris from their foraging trails, making visible highways through grass and undergrowth.  They also seem to prefer open areas such as man-made trails to march along whenever possible.  Sometimes we would share a path with these ants down the length of an entire pasture, only to part ways with no end of the ant line in sight.

Leafcutter foraging trail.

Leafcutter foraging trail.

Ant farmer – Leafcutter Ants

7 May
A leafcutter ant

A leafcutter ant (genus Acromyrmex) carries her harvest down a tree trunk.

Leafcutters in my local area are fairly restricted–we have an active colony of Texas Leafcutters (Atta texana) in the nearby Lick Creek Park, but for the most part they seem to have been driven out by invasive fire ants.  I was completely charmed by all the leafcutters we saw everywhere in Argentina.  The trails of these spiky leaf-bearing ants are easy to spot–resembling tiny parades complete with banners and floats.  The busy little ants moved about their work so fast it was hard to get a good photo of them, but managed a few.

Leaf-cutting ants are found in both the genus Atta and the genus Acromyrmex.  These ants slice off pieces of plant material and carry them back to the nest.  There they are chewed into mulch which the ants use to grow a fungus which they feed on.  Leafcutters have been farming their fungal crops so long the fungal species raised by Atta and Acromyrmex nests are unique to that environment and never found outside the ant colony.

Boys and Girls – Sexual Dimorphism in Lubbers

3 May
A small male and large female grasshopper of the same species.

Sexually dimorphic male and female grasshoppers mate.

Found this mating pair at a field site in Argentina.  We saw these guys all over.  The females of this species are huge for grasshoppers, palm-sized and heavy.  The littler male looks a bit ridiculous perched on top. My labmate Paul calls these guys the ‘chubby chasers.’

This male and female  Staleochlora exhibit a strong example of sexual dimorphism: the adult female is much larger than the adult male of the same species.  This is common in insects, where the female’s large body size allows her to hold more eggs internally.  Cut a female grasshopper open and you will see she is literally stuffed with eggs up to her thorax.  In lubber grasshoppers (family Romaleidae) adults are often so large they can no feasibly support flight, and the wings are reduced.  For this reason they are considered ‘landlubbers.’  This male lubber can still fly fairly well to seek a mate and avoid predators, but the female must rely on her cryptic coloration and limited jumping ability to avoid becoming a meal.

My mother always asks me why entomologists are so found of ‘bug sex’ pictures.  (I got her an insect field guide and many of the photos resemble the one above.)  Aside from the somewhat warped humor value of shots like the one above (well I think it’s funny) pictures like this also provide a handy side by side comparison of males an females.  Such a reference is especially important when insects are highly sexually dimorphic.  Encountered separately, the casual observer (such as myself, unaided by grasshopper specialist Paul) would probably assume the two insects above were completely different species.

Of course, a secondary reason for the abundance of photos is that mating insects are generally fairly slow and less prone to jumping away from looming camera lenses.

Palm sized female grasshopper.

Palm sized female grasshopper.