This week in Insect Morphology Seminar, we discussed several interesting papers about female genitalia and the insect ovipositor!
Steve started by telling us about germline stem cells (GSCs) in the ovaries of an earwig, Opisthocosmia silvestris. Studies in various animal species have shown that stem cells function in specialized microenvironments known as niches. Insect ovaries consist of ovarioles, which consist of the terminal filament, germarium and vitellarium. The morphology of the Drosophila GSC niche in females is known to house three types of somatic cells: terminal filament cells, cap cells, and escort stem cells. In C. elegans, the GSC niche only contains a distal tip cell equipped with long cytoplasmic structures. The female earwig GSCs are morphologically simple and consist of the terminal filament cells and escort cells; cap cells are absent and escort stem cells are not recognizable (see image below for a comparison of the three organisms).
Heather talked about how female cockroaches exchange water with their oothecae. German cockroaches usually carry their oothecae until the eggs are ready to hatch, and the success of embryogenesis may depend on water-balance between the adult female and the developing ootheca. Since oothecae that are detached from the female before embryogenesis is complete often cannot develop (especially in dry environments!), this led scientists to wonder how water is transferred to the ootheca during development. There is an area located on the proximal end of the ootheca that contains small pores that penetrate the escutcheon region of the covering of the ootheca to access the chorion! This may help maintain water balance between female and ootheca, and the German cockroach may represent an important evolutionary link in the transition from oviparity to ovoviviparity!
Colin discussed a very interesting paper on cryptic female choice in spiders and complex genital structures. Female haplogyne spiders of the species Opopaea fosuma have evolved some neat ways to get rid of or block sperm from males they have mated with. Female spiders have the anterior wall of the spermatheca, where sperm is stored before fertilization, heavily sclerotized with a cone-shaped hole in the upper part. Muscles attach to a transverse sclerite that bears a nail-like structure, and when the muscles contract, they press the nail into the hole of the spermatheca. When this happens, the copulatory oriﬁce is enlarged and the resulting suction probably allows deposited sperm to be emptied from the spermatheca (called “sperm dumping”). This mechanism is commonly used among females to influence a male’s chance of fathering their offspring; this is known as cryptic female choice. So what makes the females decide they don’t like a particular male’s sperm? Is he just not up to her standards?
Keith and Andrew both talked about genetic patterning in genitalia of the milkweed bug Oncopeltus fasciatus and the red flour beetle Tribolium castaneum. These two species differ in the anatomical complexity of their genitalia. Researchers found that the posterior Hox genes (abdominal-A and Abdominal-B) were required for proper genital development in O. fasciatus and they regulated Distal-less and homothorax in a similar way in both sexes. They did RNAi knockdown experiments to look at genitalia development and found that the genitalia are not homologous to appendages in Tribolium but they are Oncopeltus. Andrew found a paper that discovered that the same genes that regulate genitalia development also regulate the development of beetle horns. The results provided developmental genetic support for specific anatomical hypotheses of serial homology. The gene functions and interactions describe the developmental patterning of sexually dimorphic structures such as genitalia that have been critical to the diversification of species-rich insect groups.
Matt finished up with a paper talking about mating plugs in scorpions. There are two kinds of plugs in scorpions, sclerotized and unsclerotized (gel-like), which usually harden in the female genital tract. The researchers found a gelatinous mating plug in Euscorpius italicus that is composed largely of sperm – this was previously unknown in arachnids! It was discovered that fluid from the female genital tract causes sperm activation.
Heather gave a great mini-lecture on wax production and insect products. The lecture began with galls – there are over 13,000 species of insects that produce galls, from sawflies to cynipid wasps to adelgids and many flies and true bugs. There is extreme variation in gall morphology, as seen in this images below:
Galls provide nutrition to developing larvae and offer microclimate protection. Some gall inducing stimuli are saliva, maternal secretions, and larval secretions. They have complex external structures, and the mechanisms of how insects make galls and influence the plant’s response is still lagely unknown! Frederik Ronquist calls this the Holy Grail of science! We are hoping someone discovers how the fascinating structures are formed soon.
The next insect products discussed were wasp nests. Their external surfaces can range from smooth to spiky, and they can be constructed from mud, paper, etc. Mud wasps (sphecids, crabronids, etc.) and potter wasps (Vespidae) construct nests from mud and regurgitated water. It is thought that native Americans modeled their pottery after potter wasps nests!! Seeing this image, this isn’t too hard to imagine.
Oothecae are another evolutionary marvel produced by a few types in insect, including cockroaches, which construct their ootheca from calcium oxalate, proteins, uric acid and water, and mantids, which construct theirs from calcium citrate. The ootheca helps protect eggs from predators, microclimate, etc.
We then discussed bees wax, honeybee nests, and some crafty megachilids (leaf cutter bees) that use mud and pebbles to help camouflage their nests into rocksides (see image below). Some megachilids roll cut sections of leaves together and cut a small circle out of a leaf to seal the opening. Some Osmia bees use flower petals instead of leaves to construct the nest!
These amazing insect products made us wonder… could the product itself (like a nest, gall, or ootheca) be considered a part of an insect’s morphology? In ENT 502, we defined morphology as something that has a “form” and a “function”, and morphology is NOT the same as anatomy. Since some insect products can be diagnostic characters (and morphological characters can be diagnostic), can they be considered morphological characters? Or are they ecological characters? A tool has a form and a function though, so does that mean that a stick held by a monkey and used as a hammer could be consider part of the morphology of the monkey? Or a more realistic example – could the nest of a paper wasp be considered part of the wasp’s morphology? What do you think?