Morphology by   S u n i t a  N a n d i h a l l i

External Morphology

     The morphology of Harpaphe haydeniana and millipedes in general consist of the head, body, and telson. (Hopkin & Reed 1992). The head itself is calcified heavily in order to facilitate burrowing in leaf litter where these millipedes live. The head consists of the mouth, and sensory organs. The mouth has two parts, the mandibles and the first maxillae. The mandibles are for biting and crushing and the maxillae are used for tasting and also chewing. The head also bears sensory organs such as the antennae and Tӧmӧsváry organs. Since H. haydeniana possesses no eyes and is therefore blind, it used its antennae to feel and sense the surrounding environment. The exact function of the Tӧmӧsváry organ is unknown, but it is strongly suggested due to numerous studies that have been done that these organs are receptive to sights, smells, and sounds. (Hopkin & Reed 1992).

H. haydeniana
Figure 1. H. haydeniana mingling about the leaf litter

H. haydeniana’s body is about 1.8 inches in length (Acorn & Sheldon, 2001) and has 20 twenty segments (Hopkin & Reed 1992). The surface of the body of H. haydeniana is smooth, black, and has bright yellow spots that run along the millipede’s side. This bright coloring is a warning to any predator that it is poisonous, as it secretes cyanide which is a powerful poison. (Acorn & Sheldon 2001). Although H. haydeniana is known for its chemical defenses and bright coloring, millipedes in general are mostly known for its legs. The word millipede comes from name meaning “thousand foot”. (Acorn and Sheldon 2001). The legs of millipedes grow ventrally which requires the S-shaped structure that they have, giving them the appearance of “hanging down” from their legs rather than standing on them. (Hopkin & Reed 1992). Since H. haydeniana has so many legs (two pairs per segment), they run the risk of running into each other. To combat this, H. haydeniana moves its legs in slow, coordinated, metachronal waves that start at the back of the body and move towards the head. (Acorn & Sheldon 2001). Males tend to have longer legs than females in order to more strongly grasp the females during copulation. Besides this difference between sexes, each leg is the same length throughout the body. The cuticle of each segment of millipedes consists of a dorsal tergite, ventral sternite, and lateral pleurites. In polydesmid millipedes like H. haydeniana, each segment is strengthened by the fusion of all three of these structures. (Hopkin & Reed 1992).

Figure 2. The head consists of organs such as the antenna and mandibles, the body consists of tergites, sternites, and legs. The body of H. haydeniana ends with the telson.

The cuticle itself consists of the three layers, the very thin epicuticle, exocuticle, and endocuticle. H. haydeniana, as well as most other millipede species, have a calcified cuticle that they accumulate when they eat decaying organic material. It is also permeable to water, restricting this millipede’s habitat to humid areas. Since H. haydeniana is an arthropod, it has an exoskeleton and must do ecdysis. The timing of this molting process is under hormonal control. (Hopkin & Reed 1992). On the ventral side of body are the reproductive structures. The gonopods of adult male millipedes are located on the seventh segment and replace one or both legs. Female gonopods are internal but may be extruded during copulation. When this happens, they can be seen behind the second pair of legs. Each oviduct of female millipedes opens separately into organs called vulvae which are in separate sacs within the lumen and these are the structures that are everted during copulation. (Hopkin & Reed 1992).

The telson which is the last division of the body consist of a pre-anal segment, a pair of anal plates, and a sub-anal scale.  The anal plates form a valve that opens during defecation. (Blower 1985). 80 to 90 percent of dry food ingested by H. haydeniana is excreted as feces. The two forms of predominating nitrogenous waste in millipedes are ammonia and uric acid. Ammonia must be excreted quickly in order to avoid self-poisoning but uric acid can be stored temporarily in the midgut epithelium. (Hopkin & Reed 1992).


Internal Morphology

The digestive tract of a millipede is basically a straight tube from mouth to anus. Small pieces of dead plant material are passed into the lumen of the foregut where it receives secretions from salivary glands to moisten the food. Actual digestion takes place in the midgut, where enzymes secreted by epithelial cells break down the plant material into its simple chemical compounds. The most important site in any millipede for assimilation of nutrients is the midgut epithelium. Products of digestion are absorbed by the microvilli that border the cells in the midgut and are then intracellularly digested and passed to the liver which is a structure that is a layer of cells that surround the midgut. (Hopkin & Reed 1992). Adipocytes or lipocytes, fat cells that primarily compose of adipose tissue (specialized in storing energy as fat), serve as part of the digestive system. They function with the excretory system. (Camatini 1979). The organs involved in the excretory process include the midgut epithelium, liver, integument, exocrine glands, haemocytes, nephridial organs, nephrocytes, ecdysial glands, malpighian tubules, and the fat body. Millipedes have one pair of true excretory organs called nephridial organs and regulate excretion in these organisms. Nephrocytes are cells that take up substances in the haemolymph and which are then metabolized. Some of these products are then stored or returned to the circulation.  

The respiratory system in millipedes is very similar to that in insects. The exchange of O2 and CO2 between cells in millipedes takes place through tracheae. The tracheal system opens up via small holes in the cuticle called spiracles (there are two of these openings on each sternite which is the ventral portion of a segment of any arthropod). The openings of the spiracles on flat-backed millipedes like H. haydeniana are protected by a cuticular lattice (or a crystal-like structure). Since millipedes are larger, more sluggish animals, their respiration rates have been found to be relatively low when compared with other arthropods. (Hopkin & Reed 1992).

Millipedes, being arthropods, have an open circulatory system. This means that the blood of the animal, composed of liquid and cellular components are circulated throughout the body via the pumping action of the heart or dorsal vessel. The liquid bathes the organs in oxygen and nutrients and transports the products of metabolism to and from organs of digestion, storage, and excretion. This fluid in the body cavity is called the hemocoel. There is no distinction between blood and interstitial fluid; this combined fluid is called hemolymph. (Campbell et al. 2008). The principle sugar is trechalose and the main lipids are phospholipids in the hemolymph. The blood of millipedes also transports nitrogeneous wastes in a form that can be tolerated and can be excreted. (Camatini 1979).

References in Order of Appearance:

Hopkin, S.P. and Read, H.J. 1992. The biology of millipedes. Oxford University Press, New York, U.S.A. 

Acorn, J. and Sheldon, I. 2001. Bugs of Washington and Oregon. Lone Pine Publishing, Edmonton, Canada.

Blower, J.G. 1985. Millipedes Keys and Notes for the Identification of the Species. The Linnean Society of London, London, England.

Camatini, M. 1979. Myriapod biology. Academic Press Inc., London, Great Britain.

Campbell, N.A., Reece, J.B., Urry, L.A., Cain, M.L., Wasserman, S.A., Minorsky, P.V., Jackson, R.B. 2008. Biology. Pearson Benjamin Cummings, San Francisco, California, U.S.A.

Photo Citations:

Original drawing by Sunita Nandihalli 2013.

Wikimedia Commons 2012. <> Accessed 20 November 2013.