Correct Answer: A. Anopheles
Anopheles larvae are uniquely identified by the absence of a siphon (respiratory tube) and their characteristic parallel resting position to the water surface. Unlike other culicid larvae, Anopheles larvae lack the dorsal siphon that projects vertically from the abdomen; instead, they have simple spiracles on the 8th abdominal segment. This anatomical difference directly correlates with their resting behavior—they lie flat and parallel to the water surface, with their body axis aligned horizontally. This is a critical morphological distinction used in field identification of mosquito larvae in India, particularly during surveillance for malaria vectors under NTEP (National Vector Borne Disease Control Programme). The parallel resting position allows Anopheles larvae to feed on organic matter at the water surface while maintaining minimal visibility to predators. This feature is so distinctive that it forms the basis of rapid larval identification in endemic areas, essential for vector control and epidemiological monitoring.
Why the other options are wrong
B. Culex — Culex larvae possess a long, prominent siphon and rest at an oblique angle (45°) to the water surface, not parallel. The siphon is used for respiration while the larva hangs from the water surface. This is the classic distinguishing feature that separates Culex from Anopheles in field surveys. C. Aedes — Aedes larvae have a short siphon and rest at a steep angle or nearly perpendicular to the water surface. They are found in small, discrete water collections (flower pots, coconut shells, tyres) rather than large water bodies. The siphon presence and vertical resting posture are key differentiating features. D. Mansonia — Mansonia larvae possess a specialized siphon with teeth-like structures and rest at an oblique angle. They are unique in that they attach to aquatic plants using their siphon. The presence of siphon and non-parallel resting position exclude this option.
High-Yield Facts
- Anopheles larvae lack a siphon and rest parallel to the water surface—the gold standard field identification feature.
- Culex larvae have a long siphon and rest obliquely (45°); Aedes larvae have a short siphon and rest steeply (near-perpendicular).
- Mansonia larvae attach to aquatic plants using a specialized siphon with teeth; found in weedy water bodies.
- Anopheles larvae feed on algae and organic matter at the water surface; their parallel position aids camouflage and rapid escape.
- Larval morphology is the primary tool for rapid vector identification in NTEP surveillance and malaria control programs across India.
Mnemonics
SIPHON & POSITION Rule Anopheles = Absent siphon, Aligned parallel | Culex = Clear siphon, Clined oblique | Aedes = Acute angle (steep) | Mansonia = Modified siphon (teeth) Resting Angle Memory Anopheles 0° (parallel) → Culex 45° (oblique) → Aedes 90° (perpendicular). Think of increasing angles as you move through the alphabet.
NBE Trap
NBE may pair "siphon presence" with "Anopheles" to trap students who confuse morphology with taxonomy. The trap is that students might think all mosquito larvae have siphons—the absence in Anopheles is the discriminating feature, not presence in others.
Clinical Pearl
In Indian malaria-endemic districts, field workers use larval morphology to distinguish Anopheles breeding sites from nuisance mosquito habitats. A larva resting parallel to the water surface in a rice paddy or pond is almost certainly Anopheles—triggering immediate vector control intervention under NTEP protocols.
_Reference: Park's Textbook of Preventive and Social Medicine (Vector-Borne Diseases section); Jawetz, Melnick & Adelberg's Medical Microbiology (Mosquito morphology chapter)_