2. STEROLS: STRUCTURE AND DISTRIBUTION
135
terol may indeed be the nearly exclusive sterol of a phytophagous insect. In general, however, cholesterol is accompanied by one or possibly more sterols which unfortunately have never been fully identified.
Because of their physical properties and those of their acetates, they
have been called sitosterol or sitosterol-like products. It should be remembered, however, that such similarities no longer imply identity
with plant sterols of the order C 29 . The companion sterol might also be
22-dehydrocholesterol, whose physical properties are very similar to
those of ß-sitosterol (see under sterols). A complete knowledge of the
identity of the companion sterols will be of greatest importance to the
understanding of the role played by the sterols in the nutrition of
insects.
Contrasted with the extraordinary dearth of information on the
sterols of so large and so important a class of animals, is the rapidly
expanding knowledge of the sterol requirements of insects. At least
twenty different species of insect larvae, representative of the orders
Orthoptera, Coleoptera, Lepidoptera, and Diptera, have been found to
require sterols as growth factors, and none have yet been found which
do not. It appears at present that a dependence on exogenous sterol is
characteristic of all insects. A full discussion of this interesting phenomenon, which has recently been adequately reviewed {127, 128),
would go beyond the scope of the present survey. Suffice it to say that
the species which have so far been studied may roughly be divided
into two groups on the basis of their natural food and their sterols requirements. The phytophagous species are less specific in their requirements, and respond to cholesterol and its C-24 homologs. Dermes
vulpinus, the hide beetle, on the other hand, whose larva normallyfeeds on products of animal origin, is known to utilize only sterols
with the carbon skeleton of cholesterol (129).
If the phytophagous caterpillar of the cabbage butterfly is dependent on the C 28 - and C 29 -sterols for its food, it must possess a
mechanism for the elimination of the alkyl groups at C-24 to form
cholesterol, which is the only sterol found in the adult insect. Such
eliminations can indeed be shown to occur. The phytophagous flour
beetle, Tribolium confusum, can utilize cholesterol, ergosterol, and
sitos terol. It converts the C 28 - and C 29 -sterols into compounds tentatively identified by color reactions as cholesterol and 7-dehydrocholesterol (130). A more ingenious demonstration of the conversion of
higher homologs to C 27 -sterols has recently been given by E. D. Bergmann (131). The larvae of the fly, Musca vncina, were grown on a
special bran diet containing a C 29 -sterol, sitosterol. The pupae were
then fed to larvae of the hide beetles, Dermes vulpinus and D. cada-
135
terol may indeed be the nearly exclusive sterol of a phytophagous insect. In general, however, cholesterol is accompanied by one or possibly more sterols which unfortunately have never been fully identified.
Because of their physical properties and those of their acetates, they
have been called sitosterol or sitosterol-like products. It should be remembered, however, that such similarities no longer imply identity
with plant sterols of the order C 29 . The companion sterol might also be
22-dehydrocholesterol, whose physical properties are very similar to
those of ß-sitosterol (see under sterols). A complete knowledge of the
identity of the companion sterols will be of greatest importance to the
understanding of the role played by the sterols in the nutrition of
insects.
Contrasted with the extraordinary dearth of information on the
sterols of so large and so important a class of animals, is the rapidly
expanding knowledge of the sterol requirements of insects. At least
twenty different species of insect larvae, representative of the orders
Orthoptera, Coleoptera, Lepidoptera, and Diptera, have been found to
require sterols as growth factors, and none have yet been found which
do not. It appears at present that a dependence on exogenous sterol is
characteristic of all insects. A full discussion of this interesting phenomenon, which has recently been adequately reviewed {127, 128),
would go beyond the scope of the present survey. Suffice it to say that
the species which have so far been studied may roughly be divided
into two groups on the basis of their natural food and their sterols requirements. The phytophagous species are less specific in their requirements, and respond to cholesterol and its C-24 homologs. Dermes
vulpinus, the hide beetle, on the other hand, whose larva normallyfeeds on products of animal origin, is known to utilize only sterols
with the carbon skeleton of cholesterol (129).
If the phytophagous caterpillar of the cabbage butterfly is dependent on the C 28 - and C 29 -sterols for its food, it must possess a
mechanism for the elimination of the alkyl groups at C-24 to form
cholesterol, which is the only sterol found in the adult insect. Such
eliminations can indeed be shown to occur. The phytophagous flour
beetle, Tribolium confusum, can utilize cholesterol, ergosterol, and
sitos terol. It converts the C 28 - and C 29 -sterols into compounds tentatively identified by color reactions as cholesterol and 7-dehydrocholesterol (130). A more ingenious demonstration of the conversion of
higher homologs to C 27 -sterols has recently been given by E. D. Bergmann (131). The larvae of the fly, Musca vncina, were grown on a
special bran diet containing a C 29 -sterol, sitosterol. The pupae were
then fed to larvae of the hide beetles, Dermes vulpinus and D. cada-