Finding the opposite sex

Many animal species are known only from a specimen of a single sex - that is, the other sex has never been recorded. While filling that gap is not quite the same as finding a new species, I was excited to discover a female cricket of a species previously known from just a single male.

So, how do you go about species identification in a situation like this? Obviously, field guides and photo-matching will not be enough. Let me take you through the steps that lead me to believe that my lovely female is who I think she is: Arrolla lawrencei.

When I found this cricket in May this year I immediately recognised her as a member of the family Gryllacrididae. Species-level identification I put on the backburner - it can be a laborious, time-consuming process. This week, I figured it was time to get serious! She was long gone, back into the forest, but I had plenty of photos to work from.

First, double-check which family

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The cricket displays its massively long antennae and its ovipositor 

I met her one rainy night, in the forest, on low vegetation. The long curved ovipositor at the rear shows that it is a female. This is the structure used to lay eggs.

The combination of soft body, moderate leg length, very long antennae and the presence of soft pads on all segments of the tarsus shows that she belongs to the family Gryllacrididae, the Leaf-Rolling or Raspy Crickets.

A good view of the long, curved ovipositor and the soft pads on all segments of each tarsus – the pale-coloured last joint of the legs.

Which genus and species?

To get down below the family level, I referred to a 1990 paper by Rentz and John which reviews the Australian Gryllacrididae (ref. 1). The authors give a detailed description of all species known at that time and a key to shortcut the process of species identification. They also reclassify this group of insects - relying mainly on genitalia structure - in a way that better reflects their relatedness to each other. 

As my cricket was wingless, I could immediately exclude the four winged genera of the Gryllacrididae. This still left nine genera under consideration! I was able to place her in one of these - the genus Arrolla - by the following combination of characters: 

•  relatively small size (18mm body length)

• "chunky" body shape (not elongate)

• a heart-shaped head

• presence of a dark brown patch on the front of the head

• large, prominent median and lateral ocelli (white-coloured eyes in between the large, black compound eyes)

• stridulatory pegs in two parallel rows (arrows in image). Stridulatory pegs are tiny bumps on the side of the abdomen. The cricket rubs the inside of its leg against these to produce a raspy sound, which is thought to have a defensive function. 

• middle tibia with dorsal apical spine on rear margin (arrowed in image)

• hind femur with a few teeth near apex (arrowed in image)

• hind tibia armed dorsally with 5 or more short teeth on both margins (most posterior teeth arrowed in image)

There are 9 known Arrolla species. I was able to identify this cricket as Arrolla lawrencei and, in particular, separate it from the closely related Arrolla rotamah by the following features:

• the posterior margin of the fore tibia has 5, rather than 4 teeth (arrowed in image)

• the dorsal surface of the hind tibia has 4 rather than 5 teeth on the inner margin, placed in the middle 3/4 of the tibia and ending well before the apex (arrowed in image)

• the ovipositor is shorter, rather than longer than the hind femur 

• the ovipositor has a different shape to that of Arrolla rotomah, having an upturned tip with a more pronounced taper. The tip is also differently sculptured.

So, she matches the male description of Arolla lawrencei for many features, and her female-specific features do NOT match those of related Arolla species. Pretty convincing, to my mind. 

However, I would like to double-check by looking at a male ...

The male cricket

At this point Kerri reminded me of a pair of unidentified gryllacridid crickets we had photographed in April. In my obsession with tibial spines, median ocelli and stridulatory pegs, I had forgotten all about those!

On looking at the images of that pair it was immediately clear that the female - shown here on the right - was Arrolla lawrencei. So I now also had a male of the species!

Here he is, viewed from above. The image of the pair shows that the female is significantly larger than the male - by about 20%. This is consistent with other Arrolla species.

I won't bore you with a rehash of the distinguishing features of the species. Suffice it to say that all of the Arrolla lawrencei specific features seen in the female are also evident in this male.

However the rear of the male does warrant close attention. It is very different to the female (no ovipositor!) and is also species distinctive. 

The shape and size of the ninth tergite (asterisk), the cerci (white arrows) and the intercercal teeth (black arrows) match the description of the male Arrolla lawrencei in the Rentz and John paper. 

Biology of Arrolla lawrencei

So that was fun! What do we do next?!! 

Well of course we'd now like to learn more about how this species lives. What is its role in the forest ecosystem and how does that differ from other related species? We will be keeping a close eye out for Arrolla lawrencei in the future to deepen our understanding of this species in particular and gryllacridid crickets in general.

The biology of this group of insects is poorly understood. The type species for the genus, Arrolla rotamah lives in burrows in the sand and climbs bushes after dark to feed on flowers of Acacia and seed heads of daisies.

We found Arrolla lawrencei at night on long Gahnia leaves but we have not yet seen it feeding. The male seen in April was paying close attention to the female, suggesting they may have been about to mate. 

Rentz and John discovered the male Arrolla lawrencei at Bittangabee Bay, just 12km to the east, in habitat that is very similar to our forest. It all fits!

Reference:

Rentz, D.C.F and John, B. (1990) "Studies in Australian Gryllacrididae: Taxonomy, Biology, Ecology and Cytology". Invertebrate Taxonomy 3, 1053-1210.

After making this post, I contacted David Rentz, the researcher who originally described Arrolla lawrencei, and he confirmed my identification.

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