A caterpillar's body plan

The External Morphology of Larval Lepidopterans

I came across this really good diagram of a caterpillar's body plan so I decided to share it here. It outlines the important body features, many of which are useful for identification. All insects share a few basic features in their body plan, and since caterpillars are insects (they are the larval stage of butterflies and moths) you can see all these features on their body plan.

Image created by Michał Komorniczak (Poland)  - (Click image to enlarge) 

A – head, B – thorax, C – abdomen

1 – prothoracic shield

2 – spiracle

3 – true legs

4 – midabdominal prolegs

5 – anal proleg

6 – anal plate

7 – tentacle (anal horn)

a – frontal triangle

b – stemmata (ocelli)

c – antenna

d – mandible

e – labrum

Insect bodies are broken down into 3 main parts: the head (A), the thorax (B), and the abdomen (C).  The Class Insecta, which contains all insects, is within the Subphylum Hexapoda which literally means six-legged  and contrary to what some might think caterpillars only have 6 "true legs". Just like other insects the thorax of a caterpillar (B, and labelled in blue) has 3 segments, each with a pair of legs (3 x 2 = 6!). The body segments posterior to the thorax are part of the abdomen (C, labelled in orange), some of which have leg-like appendages called prolegs (4). The anal horn (7) is a prominent feature of Sphingidae caterpillars, but is not present in many other groups.

Caterpillar of the Day - Papilio troilus (Papilionidae)

Papilio troilus (Papilionidae)

The Spicebush Swallowtail may be the best known North American example of a caterpillar with eyespots. It has relatively large eyespots (e.g., in comparison to Papilio glaucus) with a flash of white inside the black pupil, which some have said makes the eye look moist or three dimensional (e.g., Wagner 2005). This species is charismatic enough to make the front cover of several books about biology including: Wagner 2005, and Rosenberg and Arp 2009. These caterpillars are not widely distributed within Canada, only entering the southern parts of Ontario (though I have heard whisperings that they are moving northward with warming climates). Instead in Eastern & Central Canada we mostly see Papilio canadensis, while South-western Canada picks up a few other species including Papilio rutulus, Papilio eurymedon, and Papilio multicaudata. Like the rest of the Papilio genus, the early instars of P. troilus caterpillars resemble bird droppings and look similar to early instar P. canadensis caterpillars (see photos in earlier posts here). P. troilus caterpillars feed on sassafras and spicebush foliage and have a strong propensity towards leaf-rolling.

Aside: Many Papilio caterpillars form silk pads on which they rest during the day. They hang on to these silk pads using little looks on their prolegs. By attaching parts of the leaf together with their silk many caterpillars - including Papilio troilus -  form leaf shelters inside of which they can hide from predators.

Late instar Papilio troilus in a leaf shelter [Photo: Wikipedia.org]

Hiding in a leaf shelter helps to hide the caterpillar, but leaf-shelters themselves are conspicuous and I have frequently observed insectivorous birds unravelling shelters in search of food. Yet, an unsuspecting bird that unravels one of these shelters would no doubt be startled but the imposing "head" thrust towards them upon entering. Their stronger propensity towards leaf-rolling (i.e., as compared to Papilio canadensis) makes me wonder if it comes as a result of the bigger eyespots. Maybe leaf-rolling evolved as a response to those bigger eyespots making the caterpillar more conspicuous. 


At the end of their final instar many caterpillars, including Papilio troilus, change colour. Typically this coincides with the termination of feeding, and the initiation of the wandering phase where they search for a place to pupate. In this case, the caterpillar changes from green to an orange-brown colour, but retains the eyespots.

Late final instar Papilio troilus caterpillar [Photo: Wikipedia.org]

At the Canadian National Collection of Insects (CNC) I was able to examine only alcohol-preserved specimens of Papilio troilus. As I mentioned in an earlier post, inflated specimens look much better. Alcohol specimens have the advantage of being protected from physical damage, moisture, and insects or mites that might eat them. Also, alcohol-preserved specimens are likely beneficial in cases where dissection is required at a later date to collect taxonomic or other types of data.

Alcohol-preserved Papilio troilus caterpillar from the CNC

Alcohol-preserved Papilio troilus caterpillar from the CNC

Alcohol-preserved Papilio troilus caterpillar from the CNC

Adult: In this species, one could make the case that the adult is equally as beautiful and interesting as the caterpillar. Adult Papilio troilus butterflies are themselves mimics of a related toxic species of Papilionid butterfly, Battus philenor. B. philenor is commonly called the Pipevine Swallowtail and both the caterpillar and butterfly are toxic because they feed on pipevine.

Adult Papilio troilus [Photo: Wikipedia.org] 

Adult Battus philenor [Photo: Wikipedia.org]


**Update: Check out this nice post on Papilio troilus from the Tales from the Butterfly Garden blog: 
http://lepcurious.blogspot.ca/2010/10/spicebush-swallowtail.html

Caterpillar of the Day - Lintneria eremitus (Sphingidae)

Lintneria eremitus (Sphingidae)

This is an interesting case on many fronts. I photographed inflated caterpillar specimens of this species at the Canadian National Collection of Insects (CNC) under the Latin name Sphinx emeritus, which I think was a misspelling of Sphinx eremitus. This species is commonly known as the Hermit Sphinx, and Wagner (2005) has the Latin name listed as Sphinx eremitus. The genus Lintneria is taxonomically close to the Sphinx genus; they are both part of the Sphingini tribe within the Sphinginae subfamily. The Latin names given to species change as we learn about them. Through closer examination of various physical attributes, in combination with inferences from genetic sequences, we continually refine the taxonomic placements. I do not know exactly which is the currently accepted nomenclature for the species (i.e., Sphinx eremitus or Lintneria eremitus).

This caterpillar is common in gardens, feeding on basil, mints (Lamiaceae), and sage, but also bee-balm and bungleweed (Wagner 2005). The frass (i.e., caterpillar droppings) of most sphingid caterpillars is cylindrical with 6 furrows, yet strangely these caterpillars (and some closely related species) have irregular shaped droppings (Wagner 2005).

For me, the caterpillar of this species is interesting because the final instar has a large black marking, dorsally, on its thoracic segments. Interestingly, Wagner (2005) calls this marking an "eye". This marking is distinct from other "eyespots" in that it is not paired. Whether it is functionally similar to other eyespot markings is a particularly interesting question. I would love to hear any thoughts or comments on this, please feel free to post comments below. Again I don't have live specimen photos, but here are some links:

http://bugguide.net/node/view/40976

http://bugguide.net/node/view/452499

http://www.silkmoths.bizland.com/seremitu.htm

These are my photos of the inflated specimens I observed at the CNC:

Lintneria eremitus (Sphingidae) - Inflated specimen, note the black "eye" marking

Lintneria eremitus (Sphingidae) - Inflated specimen (lateral view)

Lintneria eremitus (Sphingidae) - Inflated specimens from the CNC

Top right: Collected on Aug 8 1905

Top left: Collected in Ottawa, ON on Sept. 9 1999 of Monarda fistulosa

Adult: Females are said to be more likely observed as males fly too early in the evening to be effectively drawn towards light (Wagner 2005). Also, the adults may be confused with Sphinx canadensis where the two species co-occur.

Lintneria eremitus (Sphingidae) - pinned adult [Photo: Wikipedia.org]

Caterpillar of the day - Sphinx chersis (Sphingidae)

Sphinx chersis (Sphingidae)

Also known as the Great Ask Sphinx or Northern Ash Sphinx, Sphinx chersis was another species that I had a chance to examine at the Canadian National Collection of Insects (CNC). As you could probably guess from the name, the caterpillars of this species eat the foliage from various species with in the Olive family, including ash, lilac, and privet. According to Wagner (2005) these guys are occasionally found hanging on a leaf blade's underside and can get to 10 cm long; I observed a larger specimen at the CNC collection at 10.5 cm long.

Aside: If you are interested in caterpillars and don't have a copy of Wagner (2005) go out and get it today. The book is a fantastic guide to common and charismatic species, and is probably the best commercially available caterpillar ID guide for Canada and the USA. It has adult photos for most of the species as well.

Wagner, D. L. 2005. Caterpillarsof Eastern North America. Princeton, NJ: Princeton University Press. 

I don't have a live Sphinx chersis caterpillar photo to show you, so here are some links to good photos:

http://www.discoverlife.org/mp/20q?search=Sphinx+chersis

http://www.whatsthatbug.com/2009/09/08/possibly-northern-ash-sphinx-early-instar/

http://bugguide.net/node/view/41467

http://www.flickr.com/photos/gplimages/6968511146/

http://www.pbase.com/image/106172925

I do however have some great images of the inflated larvae from the CNC. Here you really get a sense of how well inflation preserves the larvae shape and colour.

Sphinx chersis (Sphingidae), box of inflated larvae form the CNC

Sphinx chersis (Sphingidae), collected Sept 25 1902

Sphinx chersis (Sphingidae), inflated.

Sphinx chersis (Sphingidae), inflated

Sphinx chersis (Sphingidae), inflated

...and once again a relatively drab adult.

Sphinx chersis (Sphingidae), pinned adult [from Wikipedia.org]

Caterpillar of the day - Ceratomia amyntor (Sphingidae)

Ceratomia amyntor (Sphingidae)

This species is better known as the Elm Sphinx or the Four-horned Sphinx. The first name relates to the caterpillar's diet (elm trees, but also birch, basswood, and cherry). It's second, more bad-ass, name comes from the 4 horn-like projections the protrude from the anterior end (i.e., not including the anal horn that characterizes sphingid larvae). Like many sphingids, at the end of the caterpillar life-stage they head down to the base of its host plant and pupate underground. Here is a photo of a late instar C. amyntor caterpillar:

Live caterpillar: Ceratomia amyntor (Sphingidae) [Photo from Wikipedia.org]

Looks to me like this guy would be incredibly cryptic on those big elm leaves. Apparently, there is some variation in the colour of these larvae, including brown, orange, and pink-brown. Curiously, the oblique stripes seem to match the venation pattern of the foliage, perhaps enhancing crypsis.

Below is a box of inflated specimens of Ceratomia amyntor from the Canadian National Collection of Insects (CNC). Amazingly, these specimens are from 1900-1912. 

Inflated larval specimens of Ceratomia amyntor (Sphingidae)

Specimen details for the above photo:

Left: Location unknown (Aug 27 1900)

Centre: from Bridgetown, N.S (Sept 17 1912)

Right: Collected in Ottawa, ON (July 31 1901)

I most would agree, the caterpillar's beauty far outmatches that of the relatively drab adult below:

Live adult: Ceratomia amyntor (Sphingidae) [Photo from Wikipedia.org]

For more information check out these links:

http://www.silkmoths.bizland.com/camyntor.htm

http://mothphotographersgroup.msstate.edu/species.php?hodges=7786

http://www.npwrc.usgs.gov/resource/insects/cateast/ceraamyn.htm

http://www.flickr.com/photos/grasslandsnationalparkvalmariesaskatchewan/1278114396/

Trip to the Canadian National Collection

Monday I took a trip to the Canadian National collection if Insects (CNC) at Agriculture and Agri-food Canada to see the Papiliondae and Sphingidae caterpillar specimens. They have a massive collection of adult Lepidoptera, but the curated larvae specimens for Papiliondae and Sphingidae species was limited. It was still a great experience and I did collect some good data.

Caterpillars were preserved either in jars of ethanol or "inflated". Eyespot markings were still clearly visible in after both types of preservation, and yes, by inflated I mean caterpillars that were inflated like a balloon and then dried. They are usually glued to a piece of wire extended from a typical insect pin. Other times the inflated larvae are glued to branches or leaves of their host plant which produces an aesthetically attractive look. This process actually retains the colour and body shape of the caterpillars amazingly well - many these specimens date back to 1900! Here are a few examples:

Papilio glaucus specimens (with some canadensis)

Papilio glaucus (just before pupation, inflated specimen)

Papilio glaucus (inflated specimen)

Sphinx kalmiae (inflated specimen)

Now compare those beauties to these specimens preserved in alcohol:

Papilio troilus (preserved in alcohol) 

Papilio troilus (preserved in alcohol)

Papilio glaucus (preserved in alcohol) 

I have many photos from many more specimens from the CNC and my next few posts will be a species-by-species account of those specimens. Stay tuned for a brief return of Caterpillar of the Day!