Homology Weekly

Homology weekly: Prognathy

Tuesday, February 2nd, 2010 | Ants, Comparative Anatomy, Homology Weekly, Morphology | 6 Comments

I am going to take advantage of figures I prepared for a talk I gave recently, where I had to explain a diagnostic characteristic of ants during the introduction. As I have mentioned before, ants are peculiar among wasps and bees in that their mouthparts are directed forward, rather than downward, in a condition known as prognathy (pro-, anterior, projecting; -gnathus, jaw).

Hypognathus condition in insects (left image from Wikimedia commons; right drawing modified after Snodgrass 1935)

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Homology Weekly: Compound Eyes

Wednesday, October 28th, 2009 | Ants, Comparative Anatomy, Homology Weekly, Morphology | 9 Comments

iGigantiops destructor/i (Michael Branstetter - www.antweb.org)

Gigantiops destructor (via Michael Branstetter - www.antweb.org)

The lateral eyes of adult insects (and most Arthropods) known as compound eyes, are like no other visual organs found in animals. You can think of our vertebrate eye as a simplified, one-lens photographic camera with a sensor composed of millions of light sensitive cells (and a blind spot, mind you). Well, that’s nothing. Each insects eye is composed of several small photographic cameras, each with its own lens and light sensitive cells (albeit, commonly only six of these). These units are called ommatidia (sing. ommatidium), and the image if formed by the combined information from all of them.1

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  1. To be honest, I have never know if this visual organ is called compound eye because it is composed of several ommatidia or because each ommatidium is composed of several elements. This has never disturb my sleep though.

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Homology Weekly: Mandible Adductor Apodemes

Thursday, September 24th, 2009 | Ants, Comparative Anatomy, Homology Weekly, Morphology | 7 Comments
The unusual mandible closer apodeme (left one in the pair) of species in the Odontomachini genus group. Disected out and cleared from all muscles (Scanning Electron Micrograph, Roberto Keller/AMNH)

The unusual hook-shaped mandible closer apodeme (left one in the pair) of species in the Odontomachini genus group (Anochetus emarginatus pictured here). Piece dissected out and cleared from all muscles (Scanning Electron Micrograph, Roberto Keller/AMNH)

Last August, before taking a break from blogging, I posted an impossible-to-answer trivia. It consisted of the image above depicting an unidentified mysterious skeletal piece (sclerite) in the shape of a hook, together with two key pieces of information: a) it is entirely internal; b) it comes in pairs. › Continue reading

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Homology Weekly: Tentorial Pits

The anterior tentorial pits (arrows) in a <i>Tetraponera aethiops</i> worker (Scanning Electron Micrograph, Roberto Keller/AMNH)

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.

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Homology Weekly: Petiole, Postpetiole and “Tubulation”

The isolated second abdominal segment constitute the characteristic petiole (blue) in ants. <i>Pachycondyla stigma</i> worker (Scanning Electron Micrograph, Roberto Keller/AMNH)

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
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  1. This post is dedicated to my long time friend and colleague Francisco Vergara-Silva

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Homology Weekly: Clypeus

<i>Tetraponera aethiops</i> worker showing the location of the clypeus in green (Scanning Electron Micrograph, Roberto Keller/AMNH)

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

  1. The only enclosed cavity  formed by the skeleton in vertebrates is the cranium, but there are no muscles inside it.

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Homology (Bi)Weekly: Dentiform Labral Setae

Saturday, May 9th, 2009 | Ants, Comparative Anatomy, Homology Weekly, Morphology, Taxonomy | Comments Off

Red Hot Chilli Peppers? No, dentiform setae in the labrum of <i>Onychomyrmex doddi</i> worker (Scanning Electron Micrograph, Roberto Keller/AMNH)

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.

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Homology Weekly: Mouthparts

Tuesday, April 21st, 2009 | Ants, Homology Weekly, Morphology | 9 Comments
Frontal part of the head in an <em>Anochetus emarginatus</em> worker, profile view (Scanning Electron Micrograph, Roberto Keller/AMNH)

Frontal part of the head in an Anochetus emarginatus worker, profile view (Scanning Electron Micrograph, Roberto Keller/AMNH)

This image shows the mouthparts of a trap-jaw ant in resting position. The only structures really visible are the prominent elongated mandibles (in yellow) that project forward. The rest of the pieces, laying immediately below, are retracted inside the preoral cavity.

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Homology Weekly: Metapleural Gland

<em>Tapinoma erraticum</em> worker. The rectangle shows the location of the left metapleural gland opening (Scanning Electron Micrograph, Roberto Keller/AMNH)

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.

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  1. Grimaldi, D. and D. Agosti (2000). The Oldest Ants are Cretaceous, Not Eocene: Comment. Canadian Entomologist 132(5):691-693.
  2. Yes, one can insult insects by calling them members of the Order Phthiraptera

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Homology Weekly: DNA

Wednesday, April 1st, 2009 | Ants, Homology Weekly, Humor, Molecular | 1 Comment

dnaAt any given position along their DNA sequences, ants may have any of the following four nucleic acid types: A, T, C or G. Unless we are dealing with the mitochondrial genome that is known to have quite a few A+T-rich regions in insects, in which case expect to find just those two types of nucleotides.

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And as we discussed last semester, the Army Ants will leave nothing but your bones.
- Tom Waits

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