DISSECTION OF PERIPLANETA AMERICANA (COCKROACH)
This is a common cockroach. This insect is the most suited for laboratory study due to its manageable size, abundant occurrence and being least specialized insect. It is Nocturnal and cursorial, inhabiting drain pipes and backeries.
External features of periplaneta americana (Cockroach) Procedure
For Dissection of Periplaneta americana (cockroach) take a fresh cockroach, which has been killed with chloroform vapour. Study the following features,’
- Shape: Body is elongated, bilaterally symmetrical and dorsoventrally flattened.
- Size: 28 to 45 mm in length and 8 to 10 mm in breadth.
- Pigmentation: The pigmentation or colouration is shining reddish brown with a yellowish area near pronotum and 2 dark patches over it.
- Exoskeleton : Entire body is covered by a chitinous cuticle. Cuticle consists of several sclerites. Cuticle is also found in pharynx, foregut, trachea and genital ducts. The skeletal plates of thorax and abdomen are dorsal terga, ventral sterna and lateral pleura.
- Segmentation and divisions : Body is metamerically segmented and divided into dorsal and ventral surfaces. It is regionated into (a) head, (b) thorax and (c) abdomen.
- Head: It is small, broader than long and is formed by several sclerites, such as epicranium, frons, clypeus and genae. Head contains appendages namely compound eyes, antennae and mouth parts.
- Compound eyes : Two large rounded eyes on sides. Eyes are large and sessile and made up of several visual units or ommatidia. It has mosaic vision.
- Antennae : First pair of appendages are antennae, which are many jointed and filiform, attached in front of eyes.
- Mouth parts : Composed of labrum, mandibles, maxillae, labium and hypopharynx. Head is connected to thorax by a short, slender neck and cervix.
- Thorax: It is composed of three segments namely prothorax, mesothorax and metathorax. The thorax contains
- Wing : 2 kinds Forewings : One pair. They originate from the tergum of mesothorax and are called as anterior wings or tegmina. They are flightless, dark, opaque, leathery and protective. Hind wings : One pair. They originate from the tergum of metathorax. Hind wings are thin, membranous and used in flight.
- Spiracles : 2 pairs in thorax.
- Walking legs: Thoracic appendages are walking legs in 3 pairs. Each leg is made up of coxa, trochanter, femur, tibia, tarsus, claws and pulvillus.
- Abdomen : 10 segments. In both sexes the 10th tergum bears a pair of ventrolateral, many-jointed anal cerci. But in males, in addition to anal cerci, there is a pair of unjointed anal styles on the 9th sternal plate. There are 8 pairs of spiracles or stigmata, one pair each on soft cuticle of terga and sterna.
- Head: It is small, broader than long and is formed by several sclerites, such as epicranium, frons, clypeus and genae. Head contains appendages namely compound eyes, antennae and mouth parts.
General anatomy of periplaneta americana (Cockroach) Procedure
- Take a freshly-killed cockroach for dissection, remove wings, cut off antennae and legs close to their bases.
- Hold cockroach in left hand and cut the lateral membranes between terga and sterna up to the anterior edge of pronotum.
- Lay the insect in the dissecting dish with dorsal side uppermost and pin it in abdominal sterna and coxae of legs. (Another better procedure of fixing cockroach is to float it in petridish containing hot[1]melted wax. Allow it to cool and in due course the animal will be embedded and dissection may be done.) Fix the head by pinning between mandibles. By means of fine scissors make a rectangular cut in the head around clypeus and anterior epicranium to expose two cerebral ganglia.
- Make a transverse cut along the posterior edge of the ninth segment (tergum) and gently remove other segments very carefully, so that the underlying organs and tissues are not disturbed.
- Uncoil intestine and stretch alimentary canal on one side. Remove fat bodies, tracheae and other muscles to expose internal organs. Study and draw the following parts :
- Heart: 13 chambers in number (3 thoracic and 10 abdominal narrow chambers). Note inter-segmental alary muscles.
- Alimentary canal : It is divided into three parts:
- Foregut: It comprises of mouth, buccal cavity, oesophagus, crop and gizzard. The buccal cavity, receives the common salivary duct. Crop is meant for storing food. The gizzard has chitinous lining, which is internally produced into six teeth for masticating the food and setae for straining the food.
- Mesenteron or midgut : It is a narrow duct originating from gizzard. At the junction of gizzard and midgut there are 7 to 8 hepatic or mesenteric caeca. (Their function is to increase the absorptive area).
- Hindgut or proctodaeum : It includes ilium, colon and rectum. The beginning of ileum is marked by 60-70 fine and long greenish yellow Maipighian tubules (excretory in function). Procedure : Note salivary glands, 2 in number, each present on the sides of oesophagus. Just invert the position of oesophagus and whitish glands along with reservoirs will be seen
- Tracheae : They are very much distinct by their striated and silvery appearance. Several tracheoles are present in the body tissues.
- Reproductive system : Sexes are separate. In case of males, mushroom glands are very distinct while in female ovarian lobes are clear.
- Male reproductive system consists of a pair of 3 or 4-lobed and rounded testes, 2 sperm ducts, mushroom gland made up of utriculi breviores and utriculi majores, ejaculatory duct. genital pouch, phallomeres and conglobate gland.
- Female reproductive system consists of 2 ovaries, 2 oviducts, vagina, collaterial glands and spermathecae.
- Nervous system : It has the following parts
- Two cerebral ganglia or brain lies dorsally on head.
- Single sub-oesophageal ganglion lies on lower or ventral side of head.
- Two circum-oesophageal connective, which connect cerebral ganglia and sub-oesophageal ganglia.
- Double ventral nerve cord starts from sub-oesophageal ganglion and extends up to posterior end. It consists of 9 pairs of fused ganglia (3 thoracic and 6 abdominal).
Reproductive system of periplaneta americana (Cockroach) Procedure
Unisexual. Sexes are separate. The procedure of dissection is same as in the case of general anatomy.
Male reproductive system of periplaneta americana (Cockroach)
Males are recognised by additional pair of unsegmented small anal styles, present ventrally in the 9th sternum. The male system comprises of testes, vasa deferentia, seminal vesicles, ejaculatory duct, phallic gland and mushroom-shaped gland. Testes are lobular or small follicular structures generally difficult to recognise easily.
Remove very carefully the fat bodies in the 5th and 6th segments. Testes are in form of rounded follicular, somewhat transparent structures. The specific gravity is higher than fat body and testes lobes sink in water in watch glass, while fat bodies float. The mushroom-shaped gland is very distinct and whitish in appearance. The sperms are said to be manufactured and stored in the utriculi of the mushroom-shaped gland, while cockroach remains in nymphal stage, after which the testes partially degenerate. Due to this, it may be explained why the testes are not seen in adult specimens.
Female reproductive system of periplaneta americana (Cockroach)
It consists of two ovaries. Each ovary contains 8 ovarioles or egg tubes. Each ovarian lobe consists of upper filiform germarium producing ova and lower wider vitellaria producing yolk cells. Each lobe gives an oviduct and all the oviducts combine to form common oviduct, which opens into vagina. Vagina leads into genital chamber, containing collaterial gland, gonapophyses and spermatheca