DESCRIPTION
Females: Head truncate or
slightly rounded. Labial region similar to that of H. bacteriophora.
SEM
observation revealed 6 elevated labial papillae (FIG.1);
each lip bears a single labial papilla at tip. Cephalic papillae
indistinct. The lateral lips contain a small circular amphidial opening.
Other structure in anterior region similar to those described for H.
bacteriophora. The anterior portion of esophagus encompasses all of
the stoma except the cheilorhabdions. Gonads amphidelphic, reflexed, reflexed
portion often extending past the vulvar opening. Hermaphroditic
females with sperm occurring in the proximal portion of the ovotestis;
amphimictic females with sperm collected in the proximal portion of the
oviduct. Vulva of the hermaphroditic female with slightly protruding lips,
never with a copulation plug as occurs on all mated amphimictic
females that possess a reduced vulva without protruding lips. Tail
pointed, normally wavy in outline, usually witth postanal swelling. Rectum
distinct, usually expanded and filled with bacteria when living, 3 rectal
glands surround the junction of intestine and rectum.
Males: Anterior region similar to females but smaller. Nerve ring usually on the anterior portion of the basal bulb. Four coelomocytes are especially noticeable: 1 pair occurs near the tip of the testis and another pair near the reflexed portion. Testis single, reflexed. Spicules paired and separate, slightly curved, spicule head variable, but usually well set off from the shaft and longer than wide; shaft lacking a rostrum; blade with a single rib that usually divides at the distal portion; spicule tips pointed. Gubernaculum flat, shorter than half the spicule length, with both the distal and proximal portions folded dorsally. Bursa open, usually pseudopeloderan with a fine tip extending beyond the bursal membrane; bursa with 9 pairs of papillae, 3 pairs anterior to the cloacal opening and 6 pairs posterior. Numbering from anterior to posterior, 1 is isolated from 2 and 3 (occasionally 2 and 3 are fused); 4, 5, and 6 normally form a group, as do 7, 8, and 9. The fifth and eighth pairs are usually bent outward (laterally), whereas the remainder are straight or bent inward (ventrally).
Infective juveniles: The infective stage is a third stage juvenile inside the second-stage cuticle. The second-stage cuticle is strongly ribbed longitudinally, but also shows weak cross striations and is closely appressed to the third-stage juvenile. If the infectives with tight sheath are placed directly in 70% alcohol, the third-stage juvenile contracts and pulls away from the second-stage cuticle. A minute dorsal tooth always present in lip region. In second-generation amphimictic females, the eggs hatch inside the females and develop to infective stages within the female body.
Measurements: All measurements are
in micrometers.
Hermaphroditic females (n = 15): Length= 3600 (2400-4900); greatest
width= 209 (120-333); length of stoma = 7 (5-10); width of stoma = 9 (8-1
1); distance from head to nerve ring = 162 (139-178); distance from head
to excretory pore = 209 (193-270); length of esophagus = 229 (206-269);
length of tail = 105 (95-124); body width at anus = 63 (38-86); percentage
vulva = 48 (45-50); length of eggs in body = 60 (53-70); width of eggs
in body = 40 (31-48); a = 17 (14-24); b = 15 (12-21); c = 34 (23-49).
Amphimicticfemales (n = 15): Length = 2100 (1500-2500) ; greatest width = 123 (95-140); length of stoma = 5 (4-6); width of stoma = 7 (5-8); distance from head to nerve ring = 111 (105-120); distance from head to excretory pore = 178 (158-206); length of esophagus = 160 (155168); length of tail = 86 (70-101); body width at anus = 31 (25-38); percentage vulva = 49 (47-5 1); length of eggs in body = 59 (48-70); width of eggs in body = 35 (25-41); a = 17 (15-19); b = 13 (IO16); c = 24 (18-32).
Males (n = 15): Length = 1000 (800-1100); greatest width = 47 (44-50); length of stoma =, 3 (2-4); width of stoma = 4 (36); distance from head to nerve ring = 104 (96-112); distance from head to excretory pore = 156 (139-176); length of esophagus = 128 (122-134); reflexion of testis = 138 (117-230); length of tail = 39 (35-43); width at cloaca = 26 (22-31); length of spicules = 49 (46-54); width of spicules = 6 (5-8); length of gubernaculum = 21 (17-24); width of gubernaculurn = 1.1 (0.3-1.6); a = 19 (18-22); b = 8 (7-9); c = 26 (23-31).
Infectivejuveniles (n = 15): Length = 768 (736-800); greatest width = 29 (27-32); distance from head to nerve ring = 109 (104-115); distance from head to excretory pore = 131 (123-142); length of esophagus = 155 (147-160); length of tail = 119 (112-128); body width at anus = 19 (17-21); a = 26 (23-28); b = 5.0 (4.6-5.0); c = 6.5 (6.1-6.9).
SYMBIOTIC BACTERIA: The infective-stage juveniles of H. megidis normally carry bacterial cells of Photorhabdus luminescens in their intestines. These cells are very characteristic in shape and can be found in the hemolymph of insects attacked by the nematodes. They produce a red pigment and impart a red color to the infected host. They are also luminescent and freshly killed insects or agar plates with 1-2-day old cultures are luminous in the dark.
TYPE LOCALITY: Mohican Hills Golf Course, Jeromesville, Ohio.
TYPE HOST: The Japanese beetle, Popillia japonica Newm. (Scarabaeidae: Coleoptera). Infection occurred in third-instar larvae on 17 an 18 October 1985.
TYPE SPECIMENS: Holotype (male) and allotype (hermaphroditic female) deposited in the Nernatology Collection at the University of California, Davis, California.
DIAGNOSIS:
The main difference between the infective stages of H. megidis, and the other species is body length. The average length of H. megidis infectives, as well as the average distance from the head to excretory pore and average length of the pharynx, separates this species from other described species of Heterorhabditis. In the males, the ratio of the length of the gubernaculum to the length of the spicule distinguishes H. megidis (less than 0.5) from other species. Also, the pseudopeloderan bursa, with the tail tip slightly protruding beyond the bursal rim is unique for this species.
REFERENCE
Poinar, G. O., Jr., T. Jackson, and M. Klein.
1987. Heterorhabditis megidis sp. n.
(Heterorhabditidae: Rhabditida) parasitic in
the Japanese beetle,Popillia japonica
(Scarabaeidae: Coleoptera), in Ohio. Proceedings
of the Helminthological Society of
Washington 54:53-59.
This document was constructed and is maintained by KHUONG
B. NGUYEN
Entomology & Nematology Department
University of Florida