Comparative Anatomy
The Newton of Natural History who never was
Portrait of Richard Owen (Smithsonian Institution's photostream).
I feel a lot of sympathy for Richard Owen. The more I read his work the more so. He is a fascinating dark character, both for the peculiar quality of his scientific oeuvre as well as for his eccentric persona. A true representative of Natural History in the Victorian Era.
History has certainly not be kind to him, foremost because it seems almost impossible to talk about him without reference to Darwin (as I am doing right now). This is quite understandable given the impact that the publication of On the Origin of Species had on defining the period. Problem is that, with a few notable exceptions1, he is wrongly portrayed as the leading antievolutionist of the time, his contribution to science thus construed as coming from a figure on the loser side of the debate and reduced to opponent of the Darwinians, in a type example of whig history.
I have previously wrote about Owen’s archetype and his clarification of the terms homology and analogy, concepts that form the cornerstone of comparative biology. He was indeed against the Darwinians, not because he rejected species evolution but because he thought natural selection, as an external force, was not a viable mechanism that could account for the pattern of shared structures make evident by comparative anatomy, the Unity of Type2. › Continue reading
- For example Rupke, N. 1994. Richard Owen: Victorian Naturalist, Yale University Press, New Haven, CT. and Amundson, R. 1998. Typology Reconsidered: Two Doctrines on the History of Evolutionary Biology . Biology and Philosophy. 13, 153-177. ↩
- And he was not alone. The view that natural selection is the main process behind evolution took some decades to gain the wide acceptance it has today. Peter Bowler has documented the extend to which naturalists after The Origin held alternative views to natural selection in his book The Non-Darwinian Revolution ↩
Homology Weekly: Tentorial Pits
The anterior tentorial pits (arrows) in a Tetraponera aethiops worker (Scanning Electron Micrograph, Roberto Keller/AMNH)
The head of an ant in frontal view has a couple of holes usually located in the area between the mouth and the place where the antennae are inserted. These holes look intriguing from the outside– Are they part of a sensing organ? Do they secrete a special chemical signal or defense substance through them? Are they use for breeding? The answer is more mundane than that. As I mentioned in an earlier post, most of what one sees in the outer surface of the arthropod’s exoskeleton does not have an external function, but is rather a symptom of the inside working in these wonderful machines. These particular holes mark the places where the cuticle invaginates to form the internal skeleton of the insect cranium known as the tentorium. The external holes produced by these invaginations are thus termed the tentorial pits.
Minelli on morphological homology
In the absence of specific arguments to the contrary, shared patterns of gene expression should not lead us, per se, to homologise organs that a comparative morphologist would never try to compare.[p.23]
Alessandro Minelli 2003. The Development of Animal Form: Ontogeny, Morphology, and Evolution. Cambridge University Press. Cambridge.
Homology Weekly: Petiole, Postpetiole and “Tubulation”
An isolated second abdominal segment constitutes the characteristic petiole (blue) in ants. Pachycondyla stigma worker (Scanning Electron Micrograph, Roberto Keller/AMNH)
The easiest way to know you are looking at an ant is to pay attention to its waist: if it consists of one or two nicely isolated segments you can be sure you made a positive identification. The basal condition for the family, common to all ants, is to have the second abdominal segment in the shape of a node or scale and distinctly isolated from the rest of the abdomen to form a petiole (remember that the first abdominal segment is coupled to the thorax as the propodeum). The functional advantage of such novel architecture seems to be an enhanced articulation between body segments, and thus greater mobility for a posterior part of the body that bears the ant’s weapons in the form of a sting or other specialized chemical producing organs like the acidopore.1
› Continue reading
- This post is dedicated to my long time friend and colleague Francisco Vergara-Silva ↩
Homology Weekly: Clypeus
Tetraponera aethiops worker showing the location of the clypeus in green (Scanning Electron Micrograph, Roberto Keller/AMNH)
When looking at an arthropod from our vertebrate perspective it is easy to forget that we are looking right at the animal’s skeleton. While our own vertebrate skeleton consists of a series of internal compact pieces with sponge-like cores that support an external layer of muscles and entrails (all nicely wrapped in skin), the reverse is true for arthropods. The arthropod skeleton consists of a series of external plates and hollow tubes that form enclosed spaces within which the internal musculature system attaches1. One consequence of this peculiar body architecture is that most of what we see on the outer surface of this exoskeleton is but a reflection of what is going on on the inside– minute external pits correspond to places where the cuticle folds in to form internal pillars, and innocent looking shallow furrows on the surface are large internal walls where powerful muscles originate. A simple examination of the exoskeleton, therefore, can tell us a lot about particular functions and consequently about an insect’s behavior. › Continue reading
- The only enclosed cavity formed by the skeleton in vertebrates is the cranium, but there are no muscles inside it. ↩
Homology (Bi)Weekly: Dentiform Labral Setae
Red Hot Chilli Peppers? No, dentiform setae in the labrum of an Onychomyrmex doddi worker (Scanning Electron Micrograph, Roberto Keller/AMNH)
Just as the anterior margin of an ant’s cranium can sometimes be armed with rows of dentiform clypeal setae (that is, especially modified hairs), the lid that closes the insect’s mouth called labrum can bear identical structures. The image above shows two of these specialized teeth-like pieces (in red) flanking an empty broad socket where a third piece used to be inserted.
Homology Weekly: Metapleural Gland
Tapinoma simrothi worker. The rectangle shows the opening location of the left metapleural gland (Scanning Electron Micrograph, Roberto Keller/AMNH)
The metapleural gland is the definitive character of ants. It is unique to the family. Nothing homologous or similar is found anywhere else in insects. Within the tree of life of Hymenoptera, myrmecologists agree that the appearance of this gland provides a good cutting point to marks-out ants as a monophyletic group1. You have it? You are an ant. You don’t? Sorry, you don’t qualify, get the hell out of here lousy wasp2. It is the ultimate ant synapomorphy.
- Grimaldi, D. and D. Agosti (2000). The Oldest Ants are Cretaceous, Not Eocene: Comment. Canadian Entomologist 132(5):691-693. ↩
- Yes, one can insult insects by calling them members of the Order Phthiraptera ↩
Richard Owen’s archetype
Caricature of Richard Owen. "Old Bones" Vanity Fair, March 1st, 1873.
I named this blog after the concept of the archetype as articulated by the Victorian naturalist Richard Owen (1804-1892). For Owen, the archetype was a representation that summed the most basic, most generalized structural arrangement common to all the members of a given group of organisms. Owen’s well-known and most important contribution to modern biological thought is, however, not his archetype concept but the clear distinction he provided between the concepts of analogy and homology. On his words:
Analogue.- A part or organ in one animal which has the same function as another part or organ in a different animal.
Homologue.- The same organ in different animals under every variety of form and function. (Owen, 1843: 374, 379)1
Homology is a concept that expresses the relationship between parts of organisms. It reflects the observation that we can identify a commonality of structure across the diversity of life. Homology thus forms the cornerstone of comparative biology.
- Owen, R. 1843. Lectures on the comparative anatomy and physiology of the invertebrate animals. London: Longman Brown Green and Longmans ↩
Homology Weekly: Gaster
The five segmented gaster (in yellow, arabic numeral) in a Cerapachys nitidulus worker. Roman numerals refer to abdominal segments (Scanning Electron Micrograph, Roberto Keller/AMNH)
Gaster is a morphological term that is very useful and yet imprecise for the purpose of comparative anatomy as it is currently used in ants. It comes from the Greek for “belly” and it refers to the collection of segments in the metasoma that remain after the pedicel of ants and wasps. It is the bulbous part of the body that hosts the insect viscera.
Homology Weekly: Propodeum
Abdomen of a Manica rubida worker. Roman numerals correspond to external abdominal segments. Propodeum shown in blue (Scanning Electron Micrograph, Roberto Keller/AMNH)
The propodeum is the subversive segment of the apocritan abdomen. As explained in a previous post, at some point during the evolution of Hymenoptera this first abdominal segment decided to part ways with its serial homologues and fuse with the thorax, forming a secondary tagma we call mesosoma. It is the Texas of the body’s segments so to speak.
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