Friday, October 13, 2017

Mite-y Bess Beetles

Click to enlarge
I came across several of these beetles when a rotted piece of firewood broke apart.  Their distinctive appearance makes them easy to identify as a horned passalus (Odontotaenius disjunctus),  a.k.a. Bess beetle or patent leather beetle for its shiny body.  It is a member of the Passalidae family.  It is unique in the family in its ability to stand freezing temperatures.  Unlike many insect species which are hard to separate, the horned Passalus is the only species in the family which occurs in the US north of Texas and Florida.

The horned passalus is the "largest showy beetle in the US." *  "The pronotum (back plate of the thorax) is square with a deep middle grove, separated from a deeply grooved  elytra by a deep waist."**   It has powerful mandibles capable of chewing through oak but do not bite and can be safely handled (if handling beetles is your thing).

Bess beetle ventral view - note the hairs and brown bumps under the "chin"
The horn on the head extends above the eyes and is quite obvious in a side view of the head.  There are fine golden hairs on the middle pair of legs, around the edges of the pronotum, and on the antennae.  Newly emerged beetles are a deep orange-brown but soon turn all black.

Newly emerged from its pupa
These beetles and their families live their entire lives in well rotted wood.  I call them families because they are a rather tightly knitted group.  The white grub-like larvae cannot digest wood by themselves and are fed with food which has been chewed by the adults including their parents or previous generations.  Like termites and anything else that eats wood, they require bacteria in their gut to digest the cellulose.  They acquire these bacteria by eating the feces of the adult beetles.  The larvae won't develop in sterilized rotted wood.

Adults communicate by stridulation, squeaky sounds made by rubbing body parts together.  They have at least seventeen different calls, more sounds than any other known arthropod.  I can picture a patient graduate student with a tiny microphone hovering over a rotten log, wondering what they are saying to each other.  I suspect they are saying, "Doesn't this highly educated, advanced biped have something better to do?"   Check out their stridulation at this link.  Recorded by Will, Kaiden and Hilton at the WOLF School. ****  

A Bess beetle larva (grub) is a prodigious eater, taking in lots of wood and passing most of it out, a virtual frass factory as seen in this video.  These grubs can even make their own sounds, rubbing their tiny stub of where the back leg should be against the rough area on the back of the second leg.  The tip of the reduced leg has a variety of teeth so the grub can create many different sounds.

You may have noticed the brown bumps below the chin in a photograph above.  These are mites that are frequently carried underneath the head and between the legs, occasionally climbing up on top for some fresh air.***  Many beetles carry mites as passengers.  This is particularly true of beetles that live underground or in dead vegetation. We discussed this in the blog about Sexton beetles

You "mite" be even more interested in the beetle/mite association.  If so go to Macromites Blog.  Yes, there is a group that is passionate about mites, an acquired taste to be sure but still a fascinating subject.

Like a few other beetles, when on their back on a flat surface they seem to be unable to roll over without help.  I suspect that if you spent your entire life in rotted wood chambers with something to hang on to, you would consider learning to roll over a waste of time.  When I put it back in its log, I thought I heard it squeak out a "Thanks."
A comprehensive resource is at this  University of Florida site. 
 *      Beetles, Peterson Field Guides
**    National Audubon Field Guide to North American Insects and Spiders, p. 555.

***  "Passalids almost invariably have associated mites - several families of mites are found ONLY in association with passalids, and many genera and species are similarly restricted - suggesting a very long association between the two groups.  I know of no mites that are harmful to the passalids, although there may well be some (e.g. tracheal inhabitants)."  (
**** WOLF is a Springfield Public Schools choice program in partnership with Wonders of Wildlife and Bass Pro for 5th grade students focusing on nature studies and conservation.

Kevin' s Cat Bite

Kevin Firth from the Butterfly House* sent me this story of a "cat" bite (as in moth caterpillar) which demonstrates the death defying heroism of the Butterfly House volunteers.  Fear not, it has a happy ending.  First the bite in Kevin's words:

So as I drove home from the Butterfly House, I was halfway down Seminole when I felt a creepy-crawly sensation on my left leg ('bout halfway up my thigh). I didn't pay it much attention and just shifted around in my seat and kept driving....but the sensation didn't go away, so I absentmindedly reached down to scratch and felt the unmistakable shape of a caterpillar under my jeans. I figured one of the wandering monarch caterpillars had hitched a ride while I was at the house, so I did my best not to smash it before I got home.

As I was pulling into the driveway, the mystery cat decided that my leg was a close enough match to its host plant and started nibbling my leg. I pulled in the garage and exited my vehicle as quickly as I could and in the most dignified manner I could manage double-timed it into my kitchen, being careful to close the door before kicking off my shoes and dropping my drawers at which point I discovered that my tormentor was not a monarch but a prepupal dagger moth caterpillar of some sort.

By that stage the cat had darkened and its cavorting about in my jeans had rubbed off much of its setae (hairs) so I have not been able to determine the exact species). I could only assume that it had decided that I had a wooden leg and was trying to excavate its pupal chamber in my thigh.  How or where I picked up this nascent hitchhiker I have no idea, but I left it happily burrowing into some wood chips along with several of its congeners. 
Acronicta rubicoma - Kevin Firth
Ruddy Dagger cat - Kevin Firth
Alas, I did not get a picture of the offending critter as I was most decidedly more interested at the time in interrupting its attempt to pupate in my dermis.  The cat in question was most likely Acronicta rubicoma since it was apparently feeding on hackberry (before my leg, that is).  I did, however, raise a few A. rubricoma just recently (and, having learned my lesson last year, provided them with a more appropriate pupation substrate):
Kevin Firth

Ruddy Dagger - Bob Patterson CC
The  Acronicta spp are commonly called dagger moths, as most have one or more black dagger-shaped markings on their forewing uppersides.  The Ruddy Dagger Moth, A. rubricoma is an exception, having a conspicuous dark ring marking instead, referred to by moth aficionados as a reniform spot ("a kidney-shaped mark") on the forewing.  I looked at a lot of images on Bugguide and it looks more oval than kidney shaped, but then I am a reformed gastroenterologist so what do I know!  Bob Kipfer


Friday, October 6, 2017

A Pair of Argiopes

A quick field trip to Linden's Prairie with Jay Barber,  Dr. Gina Wood of MSU, and a group of teachers-to-be yielded a cool pair of Argiope spiders.  One of the students spotted this black and yellow garden spider, Argiope aurantia, the web decorated by its typical zigzag stabilimentum whose purpose is still debated.

This isn't the only unique thing about A. aurantia.  The male literally dies of a heart attack after inserting his second sperm laden palp into the female.  The whole story is too gruesome for this blog (but you can read it here).

On close inspection one student noted that it was still holding the remains of an insect in its jaws as it hung upside down.  Under magnification I think it is the remains of a former grasshopper.  You can make out the head and eyes, the posterior pointing V of the pronotum and the remains of the front leathery wings (tegmentum).

As we were getting back on the bus, another student found this beautiful Argiope trifasciata, the common banded garden spider.  Its worldwide presence is thought to be due to accidental human introduction, to  Central & South America, Australia, the Mediterranean region, Africa, Sri Lanka, the South Pacific Islands, and China.

Our specimen was hanging from a web covered with feathery seed heads blown by the prairie winds.  As a parting gift, one student tossed a small curled up millipede into the web and the spider rapidly attacked.  It started spinning it, wrapping it tightly into a silken ball.  I tried to capture it on this video.  The first half is somewhat out of focus but you will get the idea.  When last seen, the millipede was tightly wrapped for a future dinner.
Discover Nature Schools is a curriculum created by the Missouri Department of Conservation to give students "hands-on learning, teaches problem-solving, and provides authentic and local contexts for learning."  DNS prepares teachers to teach students from pre-K through high school about Missouri’s native plants, animals, and habitats and connects them with nature.

Tuesday, October 3, 2017

WOLF Grasshoppers

The WOLF class field trip to Bull Creek last week had us talking about riparian tree identification and why trees are important.  As fascinating as I am sure the 5th graders found our insights, they couldn't compete with grasshoppers.  One hop and they were off and chasing them like a dog on a rabbit.

This drab gray grasshopper that fell prey to the quick WOLF hands is the Carolina grasshopper, Dissosteira carolinaThis is one of our largest grasshoppers and is commonly seen on bare ground like the gravel road we were walking.  Their slow and lazy flight may be mistaken for a butterfly.  They are easy to see in flight but are camouflaged on landing when its outline can disappear.

The brown grasshopper above especially caught my eye.  This is the autumn yellow-winged grasshopper, Arphia xanthoptera.  It may look drab but wait until it takes off.  It produces a crackling-snapping sound in flight called crepitation. The yellow-orange underwings flash brightly until it lands, then the grasshopper disappears in the ground colors.  Males may do this to attract females.

The flash of color is a trick similar to the goatweed leafwing butterfly when it lands and suddenly becomes a dead leaf.  Strangely enough, this bright color is an effective trick to fool predators.  The white tail of a cottontail rabbit or a deer works the same way.  Focus on the tail and when it suddenly freezes with its tail down, our eyes are still searching for the white flag.  Dirk Seemann's research on this trick is described here.

This post prompted my buddy George Deatz to sent the picture above of a really cool grasshopper his daughter Leslie photographed in Nixa.  This is a  Pine Tree Spur-throat Grasshopper, considered rare by those hunting grasshopper but occasionally found by us amateurs wandering around with cameras.  Good shot, Leslie!  

Thanks to Bugguide and Brandon Woo, an undergrad at Cornell and fellow bug nerd, I was able to get a quick identification of these species.  Bugguide volunteers monitor the photographs coming in for identification and make life much easier for me.

Sunday, October 1, 2017

Snails and Scorpions

Polygyra (Daedochilus) dorfeuilliana (8mm) and Rabdotus dealbatus (4mm)
Exploring the glade at Henning CA with the Master Naturalist class, we were looking under rocks and came across several snail shells.  This was on a barren limestone slab.  It is south facing, exposed to burning sun in summer and drying wind in any season, a hard place to make a living. 

I sent these to Dr. Christ Barnhart and got the species identified from left to right as Polygyra (Daedochilus) dorfeuilliana and Rabdotus dealbatus.  These are both air-breathing land snails, meaning that they evolved from their oceanic ancestors, losing their ancestral gills and developing a "lung" inside their mantle.  Most land snails are herbivorous, eating leaves and stems of plants as well as fungi and algae.

Turning more rocks we found a small scorpion.  It measured 10mm (2/5 inch) and tried to look dangerous by raising up its "tail."  They have large pedipalps out in front with pincers for grasping their food like their crayfish cousins.  For some strange reason, we always seem to focus on the rather small terminal segment of the tail, possibly because of its stinger.  With the exception of several southwest desert species, the sting in mildly painful, described as "... an immediate intense, localized, burning sensation with little redness or swelling; symptoms usually subside after about 30 minutes."  I guess that "mild" is in the finger of the beholder.

Small scorpions like this one are predators, eating mostly soft-bodied insects and other arachnids, including other scorpions.  Their common mating ritual is for the male to grasp the female's pincers.  This is a good strategy as like spiders, the female will occasionally show her gratitude for fertilizing her eggs by having the male for dessert.

I wanted to get a photograph to show its relative size and settled on the time honored method of posing it on my finger tip.  It took multiple trips to the refrigerator to get it to hold still.  Photographic hint:  I had it in a plastic magnifier box.  I may be old but am not that stupid!

More on scorpions is on this IPM Fact Sheet.

Monday, September 25, 2017

Tiny Hover Fly

We were talking with friends when a few visitors buzzed us and finally landed, waiting patiently for their portraits.  I recognized them as a type of hover fly, aka syrphid or flower fly, but couldn't ID them initially.  BugGuide quickly identified them as Toxomerus politus.

Jacsun and his fly
Hover flies get their name from the way they hover in front of flowers while nectaring.  To the uninitiated, and presumably to predators, they look dangerous with colors suggesting a miniature yellowjacket.  These aposematic colors of yellow and black are common among some of the Syrphids but these are harmless flies that couldn't hurt a human.  
T. politus and their larvae (aka maggots) likely feed on corn pollen, and herein lies the puzzle.  We are a long way from a corn plant, likely 3+ miles and 300 feet altitude up to the Prairie View plateau.  Yet, here there are several of these flies looking fat and sassy.
Wikipedia mentions that "with a few exceptions, hoverflies are distinguished from other flies by a spurious vein, located parallel to the fourth longitudinal wing vein."  They show in the illustration on the left.  Out of curiosity, I zoomed in on one of our T. politus and sure enough, there it was.

Syrphid flies as a class are important contributors to our ecosystem.  Many species are pollinators and some species have larvae that are predatory, eating aphids in large numbers, potential agents of biological control.

Like the news bee which we recently wrote about, this and other hover flies seem to have an attraction to humans, visiting and sometimes even landing on us.  Try to appreciate them and hold your impulse to swat.
Way more information that we need on T. politus is available at this Syrphidae Community Website.

Saturday, September 23, 2017

One Smooth Turtle

My third grade colleague Bruno grabbed this little 2" long turtle from a still segment of Bull Creek.  Its "shell" was smooth and almost flexible like leather.  I initially thought it was a midland smooth softshell, Apalone mutica mutica, based on illustrations in Missouri Turtles.  I sent it on to Brian Edmond who thinks it is a cousin, the eastern spiny softshell, Apalone spinifera.

This is one thin turtle.  This is probably a benefit when laying on the bottom, waiting for a tasty aquatic insect or crayfish to pass by.  Their young are susceptible to predators such as raccoons and herons while the adults only have to worry about humans looking for a source of turtle soup.*  They are said to have a painful bite.

A. spinifera is identified by small bumps on the edges of its carapace, seen below just behind the head.  The placement of the spines can help define the multiple subspecies, a fine detail only important to taxonomists and presumably to the turtles.  "What lovely tubercles you have my dear.  Care to go for a swim?"  With luck it is the beginning of a brief affair.

Deb and the turtle- She is the one in the back- by Chris Barnhart
Mating is almost a Victorian affair.  It occurs when they are 6-8 years old, no big rush here.  In the spring the male swims up to the female and nudges her head.  If she turns up her nose at him it doesn't matter as his nose is turned up as well.  If she accepts, he swims above and fertilizes without grabbing or holding on.  Several months later she will dig a cavity in a gravel bar and cover up her 8-30 eggs and from then on they are on their own.  They will hatch in September or even the following spring.

A. spinifera are said to mate in deep water, but without any deep pools along our stretch they must have to take their chances in the shallows.  There is no hurry as a large female (up to 19") may live for 50 years, and they are safe from us except for an occasional hungry human.
* Limits and regulations of soft shelled turtles are at this MDC site.  The limit on our section of Bull Creek is zero! has more details on the species.

Sunday, September 17, 2017

Walnut Sphinx Moth

Walnut Sphinx Moth - REK
Over the last several weeks I have photographed many different moths coming to our Bull Creek deck light.  My favorite wasn't one of the showy giant silk moths but this more humble walnut sphinx moth, Amorpha juglandis.  The picture above is its usual resting position that I had been seeing several every night.

The female moth emerges from its cocoon and spreads its wings to dry.  It also spreads its pheromone over the air to attract a male.  Once fertilized she will fly off to lay 200-300 eggs on an appropriate food plant.  Her caterpillars will eat the leaves of a variety of trees including butternut, hazelnut, hickory, hop-hornbeam, and of course, walnuts. Like the giant silk moths, they have no intestine and live less than a week.

 Walnut sphinx caterpillar with its horn -  MJ Hatfield CC
Like other moths, the caterpillars serve as a food source for lots of spiders, birds and insects.  A dramatic example is the walnut sphinx caterpillar parasitized below.  Chris Barnhart found it on a dried up leaf and we identified it by the color and its distinctive "horn" at the end of its abdomen.

Parasitoid cocoons on a dead walnut sphinx moth caterpillar - REK
The most likely parasitoid is a Braconidae. The female lays eggs on the caterpillar and the young burrow into the caterpillar which continues on with life, chewing leaves happily while the young grow in its body.  Eventually they make cocoons and the caterpillar's health fails.  They are careful to avoid killing the caterpillar until they are full grown and ready to emerge.
"Most life histories involve parasitizing hosts as diverse as aphids, bark beetles, and foliage-feeding caterpillars. Many species are egg-larval parasitoids, laying eggs within host eggs and then not developing until the host is in the larval stage. Unlike ichneumon wasps, many pupate in silken cocoons outside the body of the host and others spin cocoons entirely apart from the host.(3) Also unlike ichneumonid wasps, very few braconids use host pupae to complete their life cycles, except for fly parasitoids in Alysiinae and Opiinae." from Bugguide
Empty cocoons - click to enlarge
Unfortunately we found this one when they had all left their cocoons.  I kept it in a sealed box in case there was one left but had no luck, just like the poor caterpillar.

The sphinx moth itself is a cool creature which is able to whistle at a high pitch, out of our hearing range but heard by birds. We wrote a blog about this some years back.  The sound comes out of the most distal spiracle and I wonder if that may be why it holds its rear end up.  On the other hand, it may just be showing off.

Monday, September 11, 2017

One Cool Toad

My third grade colleague, Bruno, is a great hunting companion.  Curious, quick and built low to the ground, he is now feared by every amphibian and lizard along Bull Creek.  He caught three small toads over an hour within fifty feet of our house, not to mention a number of 3" skinks.  The little toad above was different from any I had seen before so I sent it off to Brian Edmonds who responded rapidly that it is a dwarf American toad.
"Dwarf American toads are confined more or less to the Ozarks. They are smaller than other American toads and are often reddish in coloration. The red spots are diagnostic."
Our "garden variety" toad along Bull Creek is the American toad, Anaxyrus americanus.  It turns out there are three subspecies.  The eastern American toad, Anaxyrus americanus americanus, is the one we all know and love, warts and all.
"The eastern American toad is medium-sized and has a large, kidney-shaped gland called the parotoid gland behind each eye. The pupil of each eye is horizontal. This toad may be gray, greenish gray, or various shades of brown. The dark spots on the back may encircle 1–3 warts. The belly is white with dark gray mottling. The call is a sustained, high-pitched musical trill lasting 6–30 seconds."  MDC Field Guide
"A little privacy please."  The male is considerably smaller than the female.
Toad eggs - REK
Mating begins with the long trills of the male's love song.  Breeding occurs in small ephemeral ponds or collections of water.  The photograph above was taken at a puddle left by a flood in 2011.  The female will lay 2,000 to 20,000 eggs in long double strands which develop into tadpoles over 2 months.  This is a risky business as the pond can go dry and eggs are high on the diet of many things living around the pond.

The tadpoles, like the adult, secrete a protective chemical from their skin called bufotoxin which can make predators sick.  It can be irritating to our mucous membranes and can make your dog sick but as mentions "toads have little defense against boys" who can be very destructive in breeding season.

Back to Bruno's find, the dwarf American toad, Anaxyrus americanus charlesmithi.  "It is smaller (only about 2 inches, snout to vent) and more reddish brown, with fewer, smaller, or no dark spots on the back; belly is cream colored with a few dark gray spots on the breast. " (MDC)  The parotoid gland is much less prominent as seen above.

The proper grip - note the red dots
When holding a toad or frog for photography, pinch a front and back leg on one side between your thumb and fore finger and they will stop kicking and hold still, but they still won't smile for the camera.  I taught Bruno this trick but he couldn't teach me how to pounce on them.

Extensive information on both species is at this site.

Saturday, September 9, 2017

Red-backed Black Widow

Gala sent me this picture of a black spider resembling a black widow but with red on its back.  She wisely didn't pick it up to see if there was an hour glass on its tummy.  We had written about black widows in several blogs and all the widows I have seen had pure black backs so I doubted this was a black widow.  It turns out I was wrong, not a surprise to my wife, Barb.

This is is a northern black widow, Latrodectus variolus, a different species from our southern variety, Latrodectus mactans. Here is what Bugguide says about it:
"For the northern black widow, the hourglass is distinct, but is broken(1) (whereas the southern black widow's hourglass is complete) and typically there is a row of red spots down the middle of the back and possibly some diagonal whitish bands on each side; the bands are typically observed on the younger, more juvenile widows."

Two days old
Sealed in a magnifier box - lots of tape!
A few years ago we blogged about how male black widows danced to avoid being eaten. This included the egg sacs that we raised in our house at the creek. I say "we" as Barb put up with it with a lesser degree of enthusiasm. Having raised them, I felt a degree of a paternal relationship and was conflicted about what to do with them. They handled part of the problem as their numbers declined daily due to cannibalism of their siblings. In nature, if all the spiderlings are the same size, cannibalism is less common.*

Eventually I was out voted in releasing them and they stayed sealed up for a year to ensure they were neutralized. I felt better knowing that a female will have 9 to 15 eggs sacs with 100-900 eggs apiece over her lifetime so she probably didn't miss these.

* How black widow mothers control their children’s cannibalism

Thursday, September 7, 2017

Horse Fly Larvae

Horse Fly Larva
We recently wrote about the horse fly, the big buzzing pesky devils whose females bite to obtain blood necessary to produce their eggs.  I just came across one of their kids deep in a gravel bar.

I was with Deb Finn and Nathan Dorff of MSU, exploring gravel bars along Bull Creek as research sites.  We hiked up Peckout Hollow, an ephemeral drainage that has been dry for weeks.  There we found an uprooted tree and a deep hole washed out by a previous flood.  In the bottom there was a small shallow water pool a foot in diameter.  Nathan began digging away the gravel and came up with this horse fly larva.

"The larvae are long and cylindrical with small heads and twelve body segments. They have rings of tubercles (warty outgrowths) known as pseudopods round the segments, and also bands of short setae (bristles). The posterior tip of each larva has a breathing siphon and a bulbous area known as Graber's organ." Wikipedia on Horse Fly

Horse fly larvae generally are found in freshwater streams and moist soil.  Finding this one far from the creek is a reminder that water continues to flow under apparently dry gravel bars.  They even found a small crawdad!  Nathan's interest is in studying the ecology of water under the gravel and the invertebrates living there.

My favorite way to see a horse fly.
The larvae of most horse fly species are predators, feeding on worms and insect larvae although some of the larger species will attack tadpoles, frogs and toads. Unlike a lot of fly larvae, mine seen in this video manages to get around quite well with its six stubby legs.  They can have toxic saliva that helps to subdue their prey.  Depending on the species they can go through 6 to 13 molts before forming the pupa.  There is one generation per year but some species will develop to adulthood over 2 years.*

Having recently been tormented by some of its kin, I felt no guilt in keeping this specimen to the end of its life.  It may mean one less bite on my next hike up the hollow.
*Purdue University Entomology Extension

Wednesday, August 30, 2017

News Bee

Yellowjacket hover fly, Milesia virginiensis - REK
There are a lot of yellow flies on the flowers now.  These are Syrphidea family members, aka. hover fly, flower fly, or syrphid fly.  At first glance they are confused with yellowjacket wasps but there are significant differences in shape, size and flight pattern. 

Syrphid flies hover around flowers before landing to feed on nectar and pollen.  They look dangerous but they have no bite or sting and are entirely harmless.  Not so with their larvae which are predators on garden pests like aphids, scale insects and thrips.  They can destroy hundreds of aphids overnight, and when lady beetle populations are low they become the dominate predator.

My favorite is the yellowjacket hover fly, Milesia virginiensis, seen above. It is a noisy creature and its sound and pattern is distinctive enough that I don't usually even look to identify it. At first glance its hornet body shape and loud buzzing are threatening, a great example of Batesian mimicry to avoid predators, but watch it closely and it just wants to be friends.  It is a very friendly and curious insect that seems to enjoy human companionship.  It's known as the "news bee" for its habit of hovering relatively close to your face as though it was delivering the latest gossip.  No other insect does this and its behavior is as distinctive as its markings.  It is said that it's good luck if one can get the insect to perch on a finger.*  Given their sound and appearance, it is more a mark of bravery.

M. vrginiensis has a stalking behavior in mating.  The male hangs around flowers in the morning looking for females, then moves to areas where the females lay their eggs later in the day.  For those wanting to take a stab at insect identification, has this description that you can compare with the pictures.
"Traits of the adult to watch for include: a completely yellow face; yellow to light brown antennae; yellow femora and tibiae, with the tarsi somewhat darker. Wings cloudy but unpatterned, typically darker at the apical end."
Sphecomyia vittata female - REK
Many other flower flies mimic yellowjackets, possibly because of their aggressive traits and unpredictable gang warfare.  This one wasn't hovering as usual and when I compared the markings it was a close mimic of M. virginiensis.  It fits many of the features described above but the thorax and abdominal markings are far different.  This is why amateur entomology is fun!

Toxomerus marginatus male - ID by James Trager
This specimen was obviously quite different.  I was lazy that day and sent it off to Bi-State Bugs where James Trager IDd it as a male Toxomerus marginatus.  It took me a while to discover how he could identify it as a male.  It turns out that the eyes have it.  The males have bigger eyes - they have to find the females who are hanging out in a flower bar, just waiting to be picked up.  The guys have to be able to tell the difference in sexes to avoid wasted time and embarrassing encounters.
"Like all other flies the males have bigger eyes which come closer together at the top of the head. Females have much smaller eyes, placed farther apart. Tiny eyes or ocelli are composed of single cells and are found at the top of the head in a triangle between the large compound eyes."
Now with your new-found knowledge about sexing (not sexting) a hover fly, you can see that this specimen I photographed was a female.  You are now off on your way to being an entomologist!  Just a few more years to go, but a warning.....It can be addictive.
* Kaufman Field Guide to Insects of North America