Experiment 02Arthropod DissectionOrgan Systems Display
Dissection & Display of the Cockroach (Periplaneta americana) Alimentary Canal & Double Ventral Nerve Cord
Lab Duration: 3.5 Hours • Wax Dissection Tray • Complete Digestive & Nervous Display
🎯 Aim: To humanely anesthetize an adult American cockroach (*Periplaneta americana*), perform a dorsal dissection under saline in a wax dissection tray, uncoil and display the complete digestive tract from esophagus to rectum, count and identify the hepatic caeca and Malpighian tubules, and expose the double ventral nerve cord with its 3 thoracic and 6 abdominal ganglia.
🔬 Principle & Insect Anatomy
The common cockroach (*Periplaneta americana*, Phylum Arthropoda, Class Insecta) provides the classic paradigm of organ specialization in mandibulate insects:
- Alimentary Canal (Digestive System): Divided into three embryological regions:
– Foregut (Stomodeum, Ectodermal, Chitin-lined): Consists of the pharynx, narrow esophagus, large sac-like crop (occupies the thoracic cavity, functions for temporary food storage), and muscular gizzard (proventriculus) lined internally by circular muscles and 6 sharp chitinous teeth that mechanically pulverize food.
– Midgut (Mesenteron, Endodermal, Absorptive): At the foregut-midgut junction, 8 blind tubular hepatic (gastric) caeca project outward, secreting digestive enzymes (amylases, maltases, proteases, lipases). The midgut is lined by a semipermeable peritrophic membrane protecting the delicate microvillar epithelium from food abrasion.
– Hindgut (Proctodeum, Ectodermal, Chitin-lined): At the midgut-hindgut junction, a ring of $100 – 150$ fine, yellow, hair-like Malpighian tubules freely bathe in the hemolymph, actively extracting potassium urate and nitrogenous waste for uricotelic excretion. The hindgut comprises a short tubular ileum, wider coiled colon, and dilated rectum bearing 6 longitudinal rectal pads (rectal papillae) that reabsorb $> 90%$ of water and salts. - Central Nervous System: Consists of a supra-esophageal ganglion (brain) in the head capsule supplying compound eyes (antennae and ocelli), circum-esophageal connectives encircling the esophagus, a sub-esophageal ganglion controlling mouthparts, and a double ventral nerve cord traversing the ventral body wall, bearing:
– 3 Large Thoracic Ganglia: Prothoracic, mesothoracic, and metathoracic, innervating walking legs and wings.
– 6 Abdominal Ganglia: Segmentally arranged, with the 6th abdominal ganglion representing a large composite fusion innervating reproductive organs and anal cerci.
Figure 2.1: Dissection display anatomy of the cockroach alimentary canal showing proventricular teeth and hepatic caeca, alongside the 9-ganglia ventral nerve cord.
📋 Step-by-Step Dissection Protocol
- Anesthesia & Pinning: Anesthetize an adult cockroach using chloroform/ether vapor in a killing jar for 3 to 4 minutes. Snip off all walking legs at the coxa-trochanter joint and snip off the tegmina (forewings) and membranous hindwings. Pin the cockroach dorsal side up in a wax dissection dish containing $0.9%,text{NaCl}$ insect saline, driving two fine pins through the lateral edges of the pronotum and two pins through the terminal abdominal tergites.
- Dorsal Tergite Incision: Using fine microdissection scissors, make lateral incisions along both sides of the abdominal and thoracic tergites where the flexible pleural membranes join. Gently grasp the posterior edge of the cut tergites with fine forceps and peel them forward toward the head, snipping underlying tracheae and dorsal alary muscles.
- Removal of Fat Body: Under a stereomicroscope, gently wash away white globular fat body tissue (*corpus adiposum*) using a fine glass Pasteur pipette stream of saline, exposing the underlying shiny digestive tract.
- Alimentary Display: Locate the paired lobed salivary glands and central salivary reservoir nestled alongside the esophagus in the prothorax. Gently deflect the large crop to one side. Identify the dark, muscular gizzard, the 8 radiating finger-like hepatic caeca, the tubular mesenteron, the yellow brush of Malpighian tubules, and the coiled hindgut. Pin out the alimentary canal in a gentle “S” curve on the black wax with micro-pins.
- Ventral Nerve Cord Exposure: Once the digestive tract has been fully examined and drawn, transect the esophagus and rectum and remove the alimentary canal entirely. Scrape away ventral fat body and muscle fibers. The white glistening ladder-like double ventral nerve cord will appear running down the mid-ventral floor. Trace the 3 large thoracic ganglia and 6 abdominal ganglia under $20times$ magnification.
