The Pigeon that Laid the Blackish Egg

The process of producing and laying eggs is one of the most vital of many organisms’ lives. The success of this endeavor determines whether certain genetic characteristics are passed on to the next generation. Oviposition is practiced by many animal groups including birds, amphibians, reptiles, fish, and insects. Other strategies include viviparity and the intermediate ovoviviparity. The primary benefits of oviparity compared to other reproductive methods include shorter development time and less initial investment by the mother and thus generally more numerous offspring. While parental care may or may not be provided once the young hatch, nutrition up until that point is provided by the yolk of the egg. Eggs may be incubated by parents or placed in a suitable environment for development which provides the temperature, airflow, and other requirements of the embryos. 

Toad eggs, Killdeer Plover nest, & ray egg case. Photos taken by the author.

As the most numerous and diverse class of animals on the planet, there are many different oviposition methods of insects. However, nearly all insects have “special modifications of the abdominal apex” which “permits the eggs to be placed in crevices or cemented to leaves or other surfaces” (Whitfield & Purcell, 2012). Representatives of the four major orders, Coleoptera, Lepidoptera, Hymenoptera, and Diptera, as well as many others, have specialized ovipositors. These appendages tend to be slightly elongated, with adaptations to allow puncture of, or deposit into particular materials. Perhaps most well known are those of many Hymenopterans, which have modified over time to act in defense. The substances secreted by these ovipositors have the potential to deliver a painful sting if the insect is sufficiently disturbed. However, the majority of ovipositors are harmless to humans, and their sole function is reproductive in nature.

Requirements for location and substrate vary between orders and species of insects. This diversity in behavior is tied to other evolutionary traits such as preferred habitat and larval food source. Species in the family Tipulidae, otherwise known as crane flies, resemble large mosquitoes but despite this morphological similarity are neither biting nor vectors of zoonotic disease. Instead, they tend to oviposit in moist soil or shallow waters, in part due to the fact that their larvae feed upon decomposing organic material and diatoms (The Editors of Encyclopaedia Britannica, 2018; The Encyclopedia of Life, n.d.). The female once she has mated searches for a suitable location for her eggs; when it is found she uses her pointed ovipositor to probe into the surface of the ground or beneath the water’s surface to deposit them.

Pigeon Tremex ovipositing in a dying tree. Photo taken by the author.

There are numerous examples of such behavior, but another with which the author has personally observed is the oviposition of the Pigeon Tremex (Tremex columba). This is a peculiar species of hymenopteran which benefits from natural ecological processes for successful reproduction. Adults are active in late summer and early autumn; at the end of this time frame, females seek out dead or dying trees in which to lay their dark eggs (Stillwell, 1967). This species’ long ovipositor is perfectly suited to boring into wood. Approximately two to five eggs are laid together in the dry substrate; as they grow and mature the larvae consume the insides of the tree (Stillwell, 1967). However, another insect has coevolved with the T. columba and become a parasitoid (Cranshaw, 2013). The Giant Ichneumon Wasp (Megarhyssa macrurus) has developed an ovipositor which is capable of piercing the same trees preferred by the Pigeon Tremex and laying its eggs within paralyzed larvae (Cranshaw, 2013). Relationships like this are quite common in nature and among insects in particular. In fact, some theorize that for every Coleopteran species, there is at least one specialist parasitoid.

Research in the field of insect reproduction is continual primarily due to sheer number of species, many of which are not yet described, but also due to the complex interactions between insect species, other animals, and plants. Observation of oviposition and related behaviors gives scientists a valuable glimpse into the natural history of a species, and a greater understanding of its ecosystem.

References

Cranshaw, W. (2013). Pigeon Tremex Horntail and the Giant Ichneumon Wasp – 5.604. Retrieved from http://extension.colostate.edu/topic-areas/insects/pigeon-tremex-horntail-and-the-giant-ichneumon-wasp-5-604/

Sezen, U. (2016). Giant Ichneumon Wasp (Megarhyssa macrurus) Ovipositing. Retrieved from http://naturedocumentaries.org/3843/giant-ichneumon-megarhyssa-macrurus-ovipositing/

Stillwell, M. (1967). The Pigeon Tremex, Tremex Columba (Hymenoptera: Siricidae), in New Brunswick. The Canadian Entomologist, 99(7), 685-689. Doi:10.4039/Ent99685-7

The Editors of Encyclopaedia Britannica (2018). Crane fly. Retrieved from https://www.britannica.com/animal/crane-fly

The Encyclopedia of Life. (n.d.). Crane Flies – Tipulidae – Overview. Retrieved from http://eol.org/pages/514/overview

Whitfield, J.B. and Purcell, A. H. III (2012). Daly and Doyen’s introduction to insect biology and diversity (3^th ed.). Oxford, England: Oxford University Press.