FREE AMINO ACIDS IN INSECTS 21 
Moreover, differences in the contents of histidine and cystine in the hydrolyzates of 
various organs were noticed. 
Hemiptera 
The adult blood of the milkweed bug Oncopeltus fasciatus contains 16 amino acids, 
including f-alanine and taurine (Table I). One interesting finding reported by AUCLAIR 
AND PaTTon! is the occurrence of D-alanine in the hemolymph of this bug. According 
to the last two authors the unnatural isomer of this amino acid was not derived from 
the food. Furthermore, no D-amino acid oxidase was found in the fat body or Mal- 
pighian tubules of this insect (AUCLAIR®). The possibility that the D-alanine was 
formed by microorganisms could not be excluded. Studies on aseptic materials are 
needed to ascertain this point. 
Some information on the amino acid composition in the blood-sucking bug Rhodnius 
prolixusisavailable. HARINGTON®’ reported the presence of leucine, histidine, histamine, 
taurine, glycine, valine, phenylalanine and alanine in the dry excreta of this insect. 
Histidine occurs also in a high concentration in the hemolymph, but histamine is 
absent. Since histidine was not found in serum-fed individuals, the author concluded 
that histidine was derived from the globin of hemoglobin. Obviously histidine gives 
rise to histamine by decarboxylation. But the occurrence of decarboxylase in this 
insect has not yet been established. 
General remarks 
The previous survey shows that, in general, the aliphatic amino acids play a dominant 
part in the chemical composition of the hemolymph. This is indicated by their large 
quantities and the constancy of their occurrence (see DRILHON®). Aromatic acids 
such as tyrosine, histidine and proline appear also in considerable amounts. Tyrosine 
is known to be involved in the tanning reaction of cuticles, while proline serves as an 
important constituent of the cuticular protein (for references see CHEN’: 34). The 
work of FuKuDA e¢ al.”7 on the silkworm larvae demonstrated that the basic amino 
acids like histidine and lysine are present in a much higher quantity in the blood of 
this insect as in mammalian plasma. Free amino acids in insect hemolymph have 
been discussed in a recent review by Wyatt!. 
Tyrosine-O-phosphate has been so far detected only in Drosophila. Hydroxyproline 
and cysteine are of relatively less common occurrence. The latter amino acid can be 
easily oxidized to cystine during the preparation of samples. Methionine sulfoxide 
has been foundin the dipterous insects andin the egg of the silkworm and the grasshopper 
(see p. 118). It must be mentioned that many free amino acids identified in insects can 
be bound with other organic compounds. For instance, WESTLEY, WREN AND MIT- 
CHELL!® found aspartic acid, cystine, glutamic acid and serine in the hydrolyzates 
of phospholipids from Drosophila larvae. They reported that in early developmental 
stages, essentially all of the non-protein amino acids were found in the lipid-soluble 
fraction. The technique used in preparing samples for amino acid analysis should be 
carefully considered. 
Several amino acids like f-alanine, taurine, ornithine, a- and y-aminobutyric acid, 
which are not found in protein molecules have been identified in the blood of various 
insects. This fact suggests that these compounds are particularly involved in in- 
References p. 132/135 
