An Introduction to Medical Entomology
For educational purposes.
Please CLICK on underlined links & Images to view:
The Subphylum: Crustacea is a group of mostly aquatic, and marine, animals. Some terrestrial forms are restricted to damp sites. They may be identified by having the following characteristics:
Head Appendages. -- Two pairs of antennae located on the 1st and 2nd segments. One pair of mandibles used for crushing. Two pair of jaw-like structures called maxillae on the 1st and 2nd segments.
Origin of Appendages. --The origin of appendages is from primitively biramous structures: protopod, exppod and endopod.
Cephalothorax. -- The head and thorax are fused in many cases to produce the cephalothorax.
Carapace. --This structure covers the head and one or more thoracic segments. It is variable in size and attached only to the head and some thoracic segments. It is frequently very conspicuous.
Excretory System. -- One pair of nephridia are involved in excretion. They occur either at the base of the second maxilla or at the base of the second antenna. They are often called "green glands."
Circulation. -- The heart and some vessels may occur in the haemocoel to circulate the blood. Blood is colorless and may contain haemocyanin or haemoglobin. The blood also is able to carry much oxygen in contrast to the terrestrial arthropods where blood carries very little oxygen.
Larval Stage. --A Nauplius larva is present.
Respiration. -- This usually involves gills.
The Class: Branchiopoda, whose name means "gill-feet" includes primarily freshwater species. Some are able to swim by using their antennae. They are fee living with compound eyes and usually a carapace. The mandibular palp is rarely present. They show both primitive and specialized characters. See Inv128 for example.
The Order Phyllopoda includes the fairy shrimp, tadpole shrimp and brine shrimp. These are primarily small (5 cm.) freshwater species with gills on their legs. They differ from other crustaceans by the absence of the 2nd maxilla. Some species display up to three reproductive strategies: bisexual, parthenogenesis and hermaphroditic. The elongated tails structures are frequently as long as the remaining body.
The Order Cladocera includes Daphnia species. They are free-swimming crustaceans that tend to swim with their dorsal side up. They have a two-valve carapace that covers most of the body save for the appendages. In some members the carapace covers only the brood pouch. The head is generally separated from the body by a deep indentation. It projects forward as a beak or rostrum. On the front of the head there is a single compound eye, which is derived from two fused eyes. Most species also have a simple naupiliar eye.
Please see following plate for Example Structures of the Branchiopoda:
Plate 49 = Phylum: Arthropoda: Crustacea: Branchiopoda, Copepoda & Ostracoda
The Class Remipedia include a group of blind crustaceans that occur in coastal aquifers containing saline groundwater, with members found in almost every ocean basin. The first described remipede was the fossil Tesnusocaris goldichi (Lower Pennsylvanian), but, since 1979, at least seventeen living species have been identified with global distribution throughout the Neotropics.
Their size is variable from 10–40 millimetres long and include a head and an elongated trunk of up to forty-two similar body segments. Swimming appendages are lateral on each segment, and theyswim on their backs. They are slow-moving. They have fangs connected to secretory glands; it is still unknown whether these glands secrete digestive juices or poisonous venom, or whether remipedes feed primarily on detritus or on living organisms. They have a primitive body plan for crustaceans, and have been regarded as an ancestral crustacean group. However, least one species, Godzilliognomus frondosus, has a highly organised and well-differentiated brain, with a large olfactory area that is a common feature for species that live in dark environments. The size and complexity of the brain suggests that Remipedia might be the sister taxon to Malacostraca, regarded as the most advanced of the crustaceans
Class: Cephalocarida – horseshoe shrimp are small, 2-4 mm. crustaceans that have an elongated body and a large head, the posterior edge of which covers the 1st thoracic segment. Eyes are absent and the 2nd pair of antennae is located posterior to the mouth, which is unique for the Crustacea. The mouth is behind the upper lip and mandibles occur on either side. The 1st pair of maxillae is very small and the 2nd pair has the same makeup as the following thoracic legs. A large basal area has outgrowths on the inner side that are used for locomotion. A forked inner branch and two outer lobes, or pseudoepipod and exopod, are present. Maxillae are unspecialized. There are 10 thoracic segments and the abdomen bears a telson but no other appendages.
Class: Maxillopoda – barnacles, fish lice, etc.
Subclass: Cirripedia are the barnacles. They are the only Crustacea, which are frequently sessile. They are typically hermaphroditic. A carapace encloses the entire body and is very heavy by being impregnated with calcium salts. All species are found in the marine environment, and some are parasitic. See Inv127 for examples
Subclass: Copepoda includes the genus Cyclops. They are primarily marine organisms bud some species also occur in freshwater. They have a single median eye and swim by means of their antennae. Parasitic forms are common. See Inv125 for example.
Please see following plate for Example Structures of the Copepoda:
Plate 49 = Phylum: Arthropoda: Crustacea: Branchiopoda, Copepoda & Ostracoda
Class: Ostracoda – seed shrimp. They have the appearance of clams because their carapace is bivalved. They are very tine and live in marine and freshwater environments. In northeastern North America they are some of the first animals to appear in ponds in springtime. They differ little in form of the body but have a great variety of appendages. Their cephalic limbs are well developed and complex. There is frequently a gastric mill and usually a pair of hepatic caeca. Antennal and maxillary glands are present. Parthenogenesis is common in some species males are unknown. See Inv126 for example
Please see following plate for Example Structures of the Ostracoda:
Plate 49 = Phylum: Arthropoda: Crustacea: Branchiopoda, Copepoda & Ostracoda
Eight principal groups are shown as follows:
Syncarida A superorder of Melacostraca called "mountain shrimps." They lack a carapace and oostegites but have exopodites on all thoracic limbs. Eyes are sessile or stalked or completely absent. The exopod is in the shape of a whip.. They live in freshwater lakes and streams. They appear to be confined to the Southern Hemisphere. See Inv134 for example.
Order: Mysidacea. -- Primarily marine animals. They have a carapace, which covers most of the thoracic somites. They have stalked eyes. These are small normally pelagic animals. See Inv132 for example.
Order: Cumacea is a small group of marine or brackish water organisms often called "hooded shrimps." They have a uniform body plan that distinguishes them from other Crustacea. There is an enlarged carapace, a slender abdomen and forked tail. They are small ranging up to 10 mm long. They have two eyes that are often fused in some species. They have a short life span living in benthic mud and debris. They are frequently observed swarming at the water surface.
See Inv135 for example.
Order: Leptostraca are marine organisms that are principally filter feeders. Some authorities consider them to be the most primitive members of the Malacostraca. They are small, ranging to about 16 mm long. They have eight abdominal segments instead of the usual six. Their compound eyes are stalked. The carapace is very large and covers the head and thorax. Eggs hatch to produce a Manca stage immature individual. See Inv133 for example.
Order: Isopoda. -- The pill bugs or sow bugs are flattened dorso-ventrallay. Abdominal appendages are modified into gills. Both terrestrial and marine species occur. Terrestrial species are widespread and found under rocks and garden wastes where they often abound. See Inv129 for example.
Order: Amphipoda. -- Primarily marine. They do not have a carapace. The eyes are sessile and the body is usually depressed. There is much variety exhibited in the group. See Inv130 for example.
Order: Stomatopoda. -- Includes the mantis shrimp, a burrowing form. They have a shallow carapace that is fused with three thoracic somites. The 2nd thoracic limb has a large, raptorial subchela. They are all marine and live primarily in burrows. See Inv131 for example.
Order: Decapoda. -- The order is divided into three suborders which are (1) Macrura = "Big Tail" includes lobsters and crayfish, (2) Anomura = Peculiar tail" includes the hermit crab, and (3) Brachyura = "Short Tail" includes the crabs.
The lobster, Homarus americanus, is an example animal for the Malacostraca: Decapoda: Macrura. It is a marine animal that burrows along rocky ocean shores (See Plate 55)
Body Plan. -- The cephalothorax is covered by one carapace. The anterior tip bears the rostrum, which protects the eyes and other sense organs in the region. Abdominal segmentation is very conspicuous.
Body Wall. -- The exoskeleton is chitinous and covers the entire body. It also has high protein content and the calcium salts that impregnate it give it hardness.
The epidermis is derived from the hypodermis. Connective tissue does not form a thick layer. Muscles are not in a continuous tube or sheath, but are broken up into a particular segmental pattern.
The haemocoel serves for circulation of the blood, and functions by a sloshing about of the blood.
Head Appendages. -- There are six segments in the head, the most anterior of which does not bear appendages.
The 1st antennae are small, not biramous, and sensory to tactile and chemical stimuli. Statocysts in the base contain grains of sand.
The 2nd antennae are large and biramous. The exopod is scale-like while the endopod is a long major branch. It is tactile and chemical sensory. The excretory pore, or green gland, lies at the base.
The mandibles function as crushing structures. The maxillae are accessory mouthparts that function in the manipulation of food. They are sensory also. The 2nd maxilla bears a gill bailer, which bails water out of the front end of the gill chamber.
Thorax Appendages. -- There are eight segments in the thorax. The 1st, 2nd and 3rd are maxillipeds. These are leg-like appendages that are modified into extra mouthparts. They function to manipulate the food and are partially sensory.
Five pairs of pareiopods are the "walking legs." The first pair is large pincers called chelipeds; the second and third pair is small chelate pincers, and the fourth and fifth pair is nonchelate pincers.
The legs are modified into structures for defense and offense. They are pincer-like at the end (= chelate). The third and fifth pair is referred to as the coxopodite, the 3rd pair being the female genital pore and the 5th pair the male genital pore. The exopods are lost during development.
Gills are borne on the bases of the thoracic appendages.
Abdomen Appendages. -- The first pair is modified into a copulatory organ in the male, while in the female these are reduced to a vestige. [Note: in the crayfish these are the first two pairs]
The second through fifth pair of abdominal appendages are pleopods or gills that create a water current.
The sixth pair is uropods, which form the tail fan.
Food & Digestion. -- Fish and some clams and snails constitute the food. These are captured by chelipeds and food is passed on to the mouthparts and to the mouth. The mouth is ventrally located near the mandibles.
The esophagus opens into the cardiac stomach. This is a gastric mill with three teeth that is modified to chew food. The teeth are shed at molting.
The midgut is a tube that contains digestive glands or a liver.
The intestine ends in the rectum with a rectal gland and anus.
Digestion is intracellular and extracellular, with a major portion occurring in the digestive gland or liver.
Circulation. -- The heart, located dorsally, has three pair of ostia, which admit the blood. Seven arteries lead out away from the heart. They are (1) ophthalmic artery to the head, (2) antennary artery to the base of antennae, (3) hepatic artery to the digestive gland, (4) sternal artery to ventral parts of the animal, which passes between a split in the ventral nerve cord and then divides into two branches, (5) dorsal abdominal artery, (6) ventral thoracic artery, and (7) ventral abdominal artery.
The arteries simply open out into the spaces of the haemocoel.
Blood runs from the haemocoel to the gills and back to the haemocoel again via ostia of the heart. The blood contains haemocyanin and has colorless corpuscles that are believed to function in coagulation.
Respiration. -- The gills are outgrowths of the body wall and covered by a lateral flap of the carapace. The early recognition that gills are part of the exoskeleton provided evidence for their being molted. It was much later also recognized that the counterparts of gills, the tracheoles of insects, were similarly involved in the molt.
The swimmerets circulate water around the gills, and the gill bailer pulls water out the front end through the 2nd maxilla.
Excretion. -- The green glands located at the base of the 2nd antennae are involved in the excretion process. These glands are similar to nephridia but they are bathed in a haemocoel and not a coelom as is true of nephridia.
Support & Protection. -- The exoskeleton is very hard and rigid, while the pincers are formidable defense weapons.
Locomotion. -- Homarus can only walk; there is no swimming of adults as the body is too heavy. The abdomen pushes the animal along backwards.
Sense Organs. -- There are sense organs located on the 1st and 2nd antennae, as well as on the mouthparts. The compound eyes are well developed and consist of a large number of ommatidia with square ends. Each ommatidium consists of a cornea, pigmented cells, a lens and a nerve element.
Statocysts are hollow cavities with sensory cells at the base of the 1st antennae. They are an inpocketing from the surface (ectoderm and cuticle on the inside). Molting results in a loss of statoliths (sand grains), which must be reacquired after each molt.
Nervous System. -- The system is very similar to that of the Annelida. There is a dorsal brain, circumesophageal connectives, and a double, solid, ganglionated ventral nerve cord. The brain is larger and more subdivided, however. Fusion of ganglia is in the anterior part of the body.
Reproduction. -- The sexes are separate in Crustacea. Genital openings are located on the 3rd peripod in the female and the 5th peripod in the male. The annulus ventralis is the seminal receptacle of the female located on the posterior part of the 5th periopod, and is not connected with the genital tract.
In the male a sperm guide is located on the first pair of pleopods, which have become modified. Fig. Inv123 shows the relative positions of the heart, testes and gonoduct.
Sperm is stored in the seminal receptacle of the female, and they are liberated at egg laying time. Eggs are attached by a sticky secretion to the swimmerets of the female and undergo most of their development there. Most larval stages are in the egg. Mysis larvae occur that swim until after the third molt.
Most crustaceans are aquatic, being dominant in the sea but also occurring in lakes, ponds and streams. A few species such as pill and sow bugs are terrestrial in damp environments. Terrestrial crustacea fluoresce with various colors under ultra-violet light. Shrimp, lobsters and crawfish that include most of the group, and their habits in the aquatic environment are similar to that of insects on land. Most species are free-living. They prey as scavengers on animals that inhabit aquatic plants. Many of the species occur in very large numbers. The smaller species are important food for larger aquatic animals. However, only a few species of crustaceans are important in transmitting human parasites (Matheson 1950).
Two groups, the Malacostracea and Entomostraca, have members that are intermediate hosts of human parasites.
These are the larger crustaceans such as crabs, lobsters and crayfish. Among the several orders, only the Decapoda act as intermediate hosts of human parasites.
Included here are very large numbers of small fresh water and marine species, some of which have a parasitic life. Their importance as the main food supply of larger aquatic animals such as fishes is great. Among the four orders, Cladocera, Phyllopoda, Ostracoda, only Copepoda, only the Copepods are intermediate hosts of human infections.
Some of the Crustacea serve as intermediate hosts of human parasites, their involvement being known from Biblical times (Matheson 1950). The larval stage of the Guinea worm (Dracunculus medinensis) occurs in copepods of the genus Cyclops. Animals and humans are infected from the drinking water. When the infected Cyclops are swallowed the Guinea worm larvae exit and penetrate the mammal's intestinal tract. They move through the tissues ending up in cutaneous connective tissue. After 10-14 months adult female worms may reach over 100 cm. in length. They produce blistered lesions mainly on the lower parts of the body. The offspring of the females are emitted from lesions that break when the mammal enters fresh water. There are few symptoms in the mammal until the worm females are mature, making it difficult to control an infestation. The late-appearing symptoms in animals include itching, vomiting, diarrhea and unrest. Distribution of the Guinea worm is now widespread in both hemispheres (Matheson 1950).
Lung fluke of the Far East and the Neotropics, Paragonimus westgermani, larvae infest freshwater crabs and crayfish. The lungs of mammals including humans become infected. In the life cycle a snail (Melania) and a freshwater crayfish or crab are involved. Humans become infected when consuming raw crabs or crayfish raw. Usually the parasite passes from the lungs to other body organs and even the brain. Symptoms vary depending where the parasite ends up in the body.
An important parasite of humans is the broad fish tapeworm, Diphyllobothrium latum, which is common in North America. The larval stages develop in freshwater crustaceans Diaptomus and Cyclops. Infected crustaceans are consumed by fish after which the larvae penetrate the stomach wall. After a few days the larvae attach to the tissues where they remain in an encysted state called "plerocercoids." Consuming uncooked fish results in human infestation. Tapeworms mature in about six weeks after ingestion. They are widely distributed throughout the world. Infections can result in severe anemia and weakness (Matheson 1950).
Key References: <medvet.ref.htm>
Brusca, R. C., and Brusca G. J. 2003. Invertebrates. Second Edition. Sunderland, Massachusetts: Sinauer Pub.
Brusca, R. C. & G. J. Brusca. 2003. The Crustacea. In: Brusca R.C. & G. J. Brusca, Invertebrates. Sunderland, MD: Sinauer Associates
Ferrari, F., Fornshell, J., Vagelli, A., Ivanenko, V., and Dahms, H. 2011. Early Post-Embryonic Development of Marine Chelicerates and
Crustaceans with a Nauplius. Crustaceana 84 (7): 869–893.
Matheson, R. 1950. Medical Entomology. Comstock Publ. Co, Inc. 610 p.
Service, M. 2008. Medical Entomology For Students. Cambridge Univ. Press. 289 p
Legner, E. F. 1995. Biological control of Diptera of medical and veterinary importance. J. Vector Ecology 20(1): 59-120.
Legner, E. F.. 2000. Biological control of aquatic Diptera. p. 847-870. Contributions to a Manual of Palaearctic Diptera,
Vol. 1, Science Herald, Budapest. 978 p.
Turner, J. 2004. The Importance of Small Planktonic Copepods and Their Roles in Pelagic Marine Food Webs. Zoological Studies 43(2): 255–266