LIBRARY

EPHEMERIDA 217
but continuous flight. The wings are somewhat triangular in shape, the fore
pair greatly developed and the hind pair very small or even absent. The venation
is of a primitive type, as indicated by the accompanying drawings (Figs.
76, 80). One of the important features of the venation is the presence of the intercalary
veins which have no basal connections but are supported by numerous
cross veins. The legs are variable, being well developed in some species and
atrophied in others. In the males of many species the [are pair is much longer
than the others and may be extended forward when at rest, appearing not unlike
antennre. The most important anatomical features of the naiads and adults
may be tabulated as shown on page 215.
Internal anatomy. - The internal anatomy of mayflies is only imperfectly
known in spite of the fact that the great naturalist Jan Swammerdam rather
carefully worked out the internal anatomy as early as 1666. An unusual organ,
not found in other insects, is a small, wholly chitinous body located in the head,
just below and posterior to the ocelli. It lies between two tracheal tubes that
connect on each side with the main tracheal trunks extending into the head.
It is called Palmen's organ and occurs in the naiads and adults, and, while
thought to have something to do with orientation in the immature forms,
its function in the imagos is unknown. The more important known
features are:
Naiad Adult
Alimentary canal for normal processes of Alimentary canal modified for the storage
digestion. of air to aid in flight.
CEsophagus wide. CEsophagus narrow tube with dilator
muscles to regulate the air flow from
mouth to stomach.
Stomach with normal epithelial and muscu- Stomach an air storage sac.
lar coats.
Malpighian tubules numerous, may be more Malpighian tubules 40 in number.
than 100.
Fore part of the hind intestine with
valves to check the air flow from the
stomach.
Sexual organs primitive with no accessory
glands, gonoducts paired in both sexes,
each with separate opening.
The order EPHEMERIDA is a comparatively small one, consisting of about
150 genera and some 1,270 species distributed in the Palrearctic, Nearactic,
Neotropical, Ethiopian, and Indo-Australian regions. 'Probably less than half of
the species of the world are described to date.