INSECTS – DRY MOUNTS

Dytiscus Adhesive Foot Pad
Found on the underside of a diving beetles’s foot

A crown of golden setae (bristles) surround two large suction cups formed from modified chitinous bristles. Adjacent to the large suction cups is a field of small setae having bulbous ends which, when living, can invert, forming suction cup-like terminal ends.


The male diving beetle, Dytiscus, has two front legs equipped with adhesive pads designed explicitly for latching on to a female diving beetle’s streamlined body to secure the two together for underwater mating. The female’s legs do not carry these specialized structures, so females never try to grab males. Instead, the female’s body is counter-adapted for avoiding capture. The outside shell of a female diving beetle is lozenge-shaped and embossed with subtle grooves. They are slippery and tough to hold on to, even when solidly grasped by a naturalist’s fingers. To get a leg-up on female slipperiness, male diving beetles ambush females as they pass by from hiding places among floating vegetation. When unexpectedly seized, the supprised female swims forcefully and erratically, colliding with floating debris and occasionally crashing into the pond’s bottom. Most times, the female dislodges the male – but not always. If females could always dump their clingy suitors, the species would have long ago become extinct. It seems that female diving beetles are not struggling to avoid all mating but rather to control it. The fight between the two sexes is over which sex has the power of mate selection.

     The competitive mating struggle between the sexes has resulted in an evolutionary arms race. The outcome of the violent premating encounters will determine the genetic heritage the progeny carries. Will the male beetle having the most robust adhesive pads be successful? Or will the genetic lottery favor the females with the slipperiest shells?  If the male controls the battle, selective heredity will select male diving beetles with more powerful suction cups. Alternatively, do the best traits come from those mothers having cast-off the most suitors? If that is the case, being the slipperiest female is the winning mate-selecting strategy. A victory that will genetically nudge descendent females into being even slipperier.

         An important ecological rule is that if a commonly practiced mating process harms the female, then there must be a counterbalancing benefit to the survival of her progeny. If female diving beetles have to suffer throughout their reproductive lifetime of being repeatedly ambushed, negative consequences, such as being caught by a fish or getting stuck in the mud, must accumulate. Assuming the ecological rule applies to diving beetles, not-yet-understood benefits must pass on to the diving beetle’s offspring.

Legs of Dytiscus marginalus Dry mount
The font on the printed label suggests the slide was made by Watson and Sons, London

Entire adhesive pad on the underside of the leg from the slide pictured above

 

The underside of a male Dytiscus marginalis showing the position of its legs

Back of a Dytiscus water beetle

Photo Notes: The microscope slide was illuminated with three diffused top-stage LED lamps. Four overlapping images stacks of sixty images each were processed using Zerene Stacker. The final image stacks were then stitched together using Adobe’s Photomerge. The slide’s asphaultium black background was retouched to yield an even blackness. Nikon BD Plan 10X/0.25 objective, Nikon 2.5X Photo Projection eyepiece, Nikon Labophot II microscope, Canon EOS D5 Mark II at ASA 100.



Head of Tipula oleracea
Smith and Beck, London 

c 1850

 

The slide carries the name of England’s most commonly found crane fly, Tipula oleracea, but the head cemented beneath the glass belongs to the less common species Tipula paludosa. Tuffen West, a skillful biological illustrator of many nineteenth-century papers, magazines, and books, complained that microscope slide preparers were becoming lax in supplying accurate scientific names of their mounted specimens. He further lamented that even when they did so, they were frequently incorrect. (1) Smith and Beck’s slide, Head of Tipula oleracea, is an example of such an error. Mistakes, however, can be of value to science historians by providing an opening through which to probe what was going on during the slide making process.

      Adult crane flies look mosquito-like but are much larger, and fly slowly while allowing their extremely long legs to dangle. They occasionally enter homes on summer nights as they seek out one another to mate. Even though crane flies are harmless to humans, their giant mosquito-like appearance gives the power to startle. The two English crane flies associated with this slide look identical when casually viewed. Both species have similar larval stages colloquially called leatherbacks. The larvae occasionally appear in tremendous numbers in the soil of habitually overwatered lawns. They feed upon the roots of grasses and are considered to be lawn damaging pests in England. Unfortunately, both species are accidental introductions to the American northeast.

                One difference between the two species of crane flies is their reproductive cycle. T. oleracea adults reproduce twice a year, once in early summer and again in the early autumn.  T. padula nuptial activities occur only once a year in early autumn.  T. oleracea goes through two generations a year while T. padula, only one. The maker of this slide, most likely complacent by single species summer collecting, never noticed the second species mixed in with the autumnal catch. Confusing the two species in a bug net is understandably easy, but the defining difference between the two becomes clear when viewing the insects’ heads under a microscope. T. oleracea’s eyes almost touch each other in the middle of the face giving the fly’s portrait a cross-eyed look. T. padula’s eyes are widely separated. 

The Slides Maker

James Smith (c. 1800 – 1870) and Richard Beck (1827 – 1866) owned a London optician shop, which was in business as Smith & Beck from 1843 to 1865. The company manufactured microscopes and produced prepared microscope slides.  The paper wrapping the slide shows a small “B” in a triangle on the right side of the specimen and the letter “S” (covered by the oval specimen label) on the left. Smith and Beck used this design as their trademark for all their in-house prepared microscope slides. In addition to selling prepared slides, Smith and Beck also sold the supplies needed by amateur microscopists to make slides of their own. One of the materials they sold was the decorative printed-paper sheets for slide wrapping, some of which included their trademark. For this reason, microscope slide collector/historians cannot always be certain if a slide carrying the “S. B.” trademark is one actually prepared by Smith and Beck Opticians.

      Assessing a finished slide’s quality is one factor used to attribute the origin of an “S and B” paper-wrapped slide to Smith and Beck’s optician shop. Another is being able to find other slides of similar specimens to affirm constancy of mounting style. The microscope slide Head of Tipula oleracea was one listed in Smith and Beck’s slide advertisements and, judging from the number of currently surviving slides – it was a popular seller. Comparing the handwriting on the specimen label to other slides by the same company clinches the assumption that this is an original Smith and Beck prepared slide.

      The Smith and Beck’s Head of a Crane Fly slide is an example of dry mounting with pressure. The first step in the mounting process is to “clear” the specimen. Clearing is done chemically by soaking the dead insect for about a week in a solution of potassium hydroxide. The caustic chemical dissolves away muscle and other soft internal tissues leaving only its exoskeleton and other chitinous parts. All traces of potassium hydroxide need to be removed, along with other insoluble particular debris no longer held in place. Changing baths of rainwater or distilled water, usually over several days, ensured no corrosive chemical would be left to continue acting on the remains after being mounted. (2) 

      After severing the head from the insect’s now clean and translucent body, it is placed on a microscope slide while still soft and wet. A fine needle is used to position the delicate antenna and mount appendages. A glass coverslip is gently placed over the head and a small weight placed atop to flatten it and hold everything in place as it drys. The edges of the coverslip are glued to the microscope slide using lacquer or gold-size.  The sealing is to protect the specimen from picking up moisture from the air.  Finally, the entire ensemble is wrapped-u with decorative slide-paper having a hole cut in the right place to allow lighting and viewing the crane fly head. Many dry mounts prepared this way are still in perfect condition almost two hundred years later!

Photographic information

The actual length of the Tipula head is six millimeters (approximately ¼ inch) with a final seventeen by twenty-six inch exhibition print yielding a linear magnification of sixty-eight times or a seven-thousand increase in surface area. The photograph is a combination of sixteen processed focus stacks. Each stack of fifty refocused images was shot using a ten powered objective. The result was a total of eight-hundred individual exposures. Zerene Stacker was the computer program used to process the image stacks. The compilated Zerene images were finally assembled into one picture using Photoshop. The combination of the two methodologies is referred to by photomicrographers as “stack and stitch.” The background was darkened in Photoshop to improve viewing.

Canon Mark II, ISO 200. Nikon Labophot, 5X BD Planapo objective, Nikon 2.5 PL photo-eyepiece, darkfield illumination.